This is an excellent question. Epic theatre (episches Theater) can be defined with three points. I should mentioned that Bertolt Brecht is synonymous with Epic Theatre.
First, epic theatre argues that the audience should not identify with the characters in the play. Instead, the audience should engage in self-reflection and view critically the drama on the stage. Some have called epic theatre didactic drama, that is, drama intended to teach something.
Second, against the Aristotelean understanding of a play offering a catharsis, the audience should critically analyze a play and see things like social injustice and misuse of power and effect change in the world in concrete ways. In this way, epic theatre is about social change.
Third, just as the play was a reconstruction of reality, so reality is also socially constructed. This means that society can be changed. These plays are also put on in such as way that there is no mistaking that they are not meant to resemble reality.