Discourse must take place in an ideal speech situation. Every subject with the competence to speak and act is allowed to take part in a discourse. Everyone is allowed to question any assertion whatever. Everyone is allowed to introduce any assertion whatever into the discourse. Everyone is allowed to express his attitudes, desires, and needs.
Discourse ethics are procedural rules for the public sphere.
One might say that Habermas has outlined a way in which we can create freedom for and freedom by democracy. The ethics of discourse set the stage for Habermas' larger theoretical project. This project constitutes a macro-level theory based in ideas about communication. The Theory of Communicative Action Incredible two volume set that it is, I do not expect that I will do much more than introduce the Theory of Communicative Action here. Well, that said, at the risk of being essentialistic here are the basics.