Subjectivism is the theory that as humans, we can't know everything, or even KNOWS anything for sure because everyone's thoughts are different, and people experiences events differently.
For example, it is possible that all that I see, that everything that I consider 'my reality' is actually just an illusion, a fake. A person can be paranoid, and suffering a lot from hallucinating, where nothing that they imagine, nothing that they see actually resembles reality, and thus they do not know what is real and what is not.
Or perhaps, like the Matrix, your entire sensory system is being tricked; maybe everything you see, everything you feel, and taste are actually part of one grand virtual reality, and a reality where there is very little chance to escape from. I could have been hypnotized or conned into believing something has happened which actually didn't occur, and vice versa; we cannot know for sure if everything that we hear of is for real, because people experience things differently and what maybe real for one person is not necessarily real to the next person.
The opposite of subjectivism is objectivism; the theory that says certain acts is objectively right or wrong regardless of our moral opinions. I believe that objectivism doesn't work at times, especially when it plays on the on moral concepts such as 'moral evil'; for example, are the terrorists who killed thousands of people in the World Trade Center objectively evil? Were the mass murders of Hitler, Stalin and other dictators objectively evil? I may say that I hate and find such people disgusting and if I can take any efficient action to get rid of these people in my society I will do so because I believe it is in my best interests. I may also call these people...