Subjectivism is an epistemological theory. It is a theory of knowledge, and how it is achieved. Subjectivism holds that knowledge is generated from the mind, without reference to reality. It holds that gaining knowledge about the world is done through introspection. It holds that metaphysically, the world is a figment of our imaginations. It holds that because reality is an aspect of our minds, it is affected by them.
Like most misbegotten notions, this theory is never held consistently. The effect of trying to hold it consistently would be a complete inability to interact with the world. One would just sit and wish for things to be "better", confused at why the world isn't doing its part.
No, the practitioners apply it only in certain respects, and to certain degrees. It is an act of evasion. An attempt to ignore the facts of reality. It is the claim that the mind controls particular aspects of reality, or that certain facts of reality don't exist, and can be whatever you mind wants. A common use of subjectivism is in the field of ethics. A subjectivist evades the objective nature of ethics, and pretends that he may act in any way he chooses, without consequence.
Subjectivism is a denial of reality. It is the acceptance of the Primacy of Consciousness. It is a denial of reality, and a denial of the Law of Identity. It states that "anything goes", and lets desires, whims, and emotions run rampant.