We see this all the time in the area of human relationships. What we thought someone meant was not always what they understood that they were communicating. Even in the realm of natural observation illusions can be created and the mind fooled. Postmodernism, however, goes further than this. Postmodernism denies the existence of an objective reality.
Postmodernism postulates that reality is in the mind of the beholder.
To postmodernists, we create our own truth. Every culture has its precepts and ways of looking at things. Postmodernists assume that the majority of people are conditioned by the dominant western paradigm or way of looking at things - which has served some groups well at the expense of others. In particular, white heterosexual males have profited at the expense of women, homosexuals, blacks, the disabled and other disadvantaged and minority groups. Postmodernists often challenge views of the world which they consider as sexist, racist, imperialist, fundamentalist or exploitative for whatever reasons, seeking to show the equal validity of minority perceptions and cultures.