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The philosophical meaning of idealism here is that the properties we discover in objects depend on the way that those objects appear to us as perceiving subjects, and not something they possess "in themselves", apart from our experience of them.

Subjective Idealism

George Berkeley, which held that external objects have actual being or real existence only when they are perceived by an observer but since the mind is passive in perception, there are ideas which one’s own mind does not cause. Only a mind or spirit can be a cause. “There is therefore some other will or spirit that produces them”. As such, this is not an argument for the existence of God (see PHK §§146-149), although Berkeley’s further discussion assumes that at least one mind is the divine mind. He is now in a position to distinguish ideas of sense from ideas of the imagination: “The ideas of sense are more strong, lively, and distinct than those of the imagination; they have likewise a steadiness, order, and coherence, and are not excited at random, as those which are the effects of human wills often are”. This provides the basis for both the distinction between ideas of sense and ideas of imagination and for the distinction between real things and imaginary things. Real things are composed solely of ideas of sense. Ideas of sense occur with predictable regularity; they form coherent wholes that themselves can be expected to “behave” in predictable ways. Ideas of sense follow (divinely established) laws of nature.

Quantum Physics "physical idealism"

The behavior of sub-atomic wave-particles is dependent upon the consciousness of a human observer.

Transcendental Idealism

The external world, he writes, provides those objects - noumena, of "things in themselves", the supra-sensible reality beyond the categories of human reason - that we sense as a sensory input (experience) and it is our mind, though, that processes this information about the world by applying the categories of  understanding, such as substance, causality,space and time allowing us to comprehend the phenomena. But Critics said that the law of cause and effect only applies to the phenomena within the mind, not between those phenomena and any things-in-themselves outside of the mind. we can't have access to the "things in themselves" , so what causes the phenomena?

Fichte

Our representations, ideas, or mental images are merely the productions of our ego, or knowing subject. For him, there is no external thing-in-itself that produces the ideas. On the contrary, the knowing subject, or ego, is the cause of the external thing, object, or non-ego. The phenomenal world as such, arises from self-consciousness; the activity of the ego; and moral awareness.

Schelling's Identity

There is no subject without object, and vice versa. there is no difference between the subjective and the objective, that is, the ideal and the real. Schelling publicly expressed his estimation of Spinoza and declared that nature and spirit differ only in their quantity, but are essentially identical (Identitaet). According to Schelling, the absolute was the indifference or identity, which he considered to be an essential subject of philosophy.

Absolute idealism

Hegel asserted that in order for the thinking subject to be able to know its object at all, there must be in some sense an identity of thought and being. Otherwise, the subject would never have access to the object and we would have no certainty about any of our knowledge of the world. According to Hegel, the absolute ground of being is essentially a dynamic, historical process of necessity that unfolds by itself in the form of increasingly complex forms of being and of consciousness, ultimately giving rise to all the diversity in the world and in the concepts with which we think and make sense of the world. The objective world is only the external, phenomenal form of "the Idea" be it named Spirit.

Schopenhauer

He pointed out that anything outside of time and space could not be differentiated, so the thing-in-itself must be one and all things that exist, including human beings, must be part of this fundamental unity because time and space are just concepts from the mind. He answered Kant's critics saying that we are in the world as much as anything else; our bodies are objects. My willpower is the only thing that sets my body apart from any other object; the will manifests itself in my body. Therfore the "thing in itself' is a manifistation of its inner will and since we proved that all things are one, the entire world is the representation (phenomena) of a single Will, of which our individual wills are phenomena. The ultimate reality is one universal will. It is the "will to live" that perpetuates this cosmic spectacle.

At the very core of human beings stirs a power – the Will – which is the very stuff of their inner being, yet which is as unfeeling and anonymous as the force which stirs the waves. Subjectivity is what we can least call our own. We bear inside us an inert weight of meaninglessness, as if permanently pregnant with monsters; and this, which is the action of the Will within us, constitutes the very core of our selfhood. Everything is fraught with appetite: human beings are simply walking incarnations of their parents’ copulatory instincts,and the whole of this fruitless desiring is founded in lack. ‘All willing’, Schopenhauer writes, ‘springs from lack, from deficiency, and thus from suffering.’ Desiring is eternal, whereas fulfilment is scanty and sporadic. There can be no end to the fatal infection we know as desire as long as the self endures. Only the selflessness of aesthetic contemplation, along with a kind of Buddhist self-abnegation, can purge us of the astigmatism of wanting, and allow us to see the world for what it is.
For Schopenhauer, who is considered to be a pessimistic philosopher, the tragedy of life arises from the nature of the will, which constantly urges the individual toward the satisfaction of successive goals, none of which can provide permanent satisfaction for the infinite activity of the life force, or will. Things such as an interest in the Arts, and a moral life based on sympathy, tend to alleviate the suffering experienced in people's lives. A more telling alleviation is to be found through the denial, or suspension, of the will through asceticism.

Schopenhauer and Hegel both answered Kant's critics with the unity of the subject and object as Schelling said but differed in the type of unity, Hegel said the objective world is only the external, phenomenal form of "the Idea" be it named Spirit which is the collective consciousness rising above all limitations of nature,  Schopenhauer said the entire world is the representation of a single Will, of which our individual wills are phenomena.