Subjectivism, at it's core, is the philosophical position that all morality is merely a matter of personal taste. For example, "Spanking your children is a form of child abuse," and it's counter argument are not intrinsically true or false but are, in actuality, the expression of the individual's personal preference on the matter. People brought up in the same society will naturally have similar moral standards; however, according to this simplest form of Subjectivism, individuals from separate societies with differing moralities are unable to prove that one standard is in error and the other correct.
Practically no one adopts Subjectivism in its simplest form. Instead most Subjectivists tend to develop their moral position along two themes. (1) Morality is not dependent on an individual or individual society's inclinations but rather it is based on the natural desires of mankind as a whole. According to this line of thought people value certain things more than others and our morals are an expression of that valuation. As a result when we ask what is 'good' what we are asking as Subjectivists is "What do all, or most, of the people desire?" If the majority of people desire to live unmolested and free than allowing people to do so is 'good.' The opposite condition, naturally, is bad. (2) The alternative theme ultimately relies on the individual as the arbitrator of what is 'good' or 'bad' but rational arguments to support those valuations are required just as they would be for any other pronouncement. Thus if we argue that murder is 'bad' in all cases then we must create logical justifications for that position (for example, it is wrong to murder another person because doing so deprives them of their lives and negatively affects the community by depriving it of the individual's financial, social, and economic contributions).
Let's move away from the theory of Subjectivism and towards the question of what it means when we say something is subjective. At it's simplest form, to call something subjective is to imply that the speaker's valuation of it is bound to their personal preference. Yet it can be expanded to include any discussion that seeks to use popular opinion as the logical basis for something's value. For example, to say that Star vs the Forces of Evil is the best animated show on television based exclusively on the speaker's opinion and to say that Dungeons & Dragons is the best role-playing game based on its popular appeal are both subjective statements.