Postmodernism is a way of thinking about culture, philosophy, art and many other things. The term has been used in many different ways at different times, but there are some things in common.
Postmodernism says that there is no real truth. It says that knowledge is always made or invented and not discovered. Because knowledge is made by people, a person cannot know something for sure - all ideas and facts are 'believed' instead of 'known'. People believe that they know what the truth is, but they will think that the truth is something different later. This is the opposite of 'objectivity', which says that the truth is always there and people have to discover it.