Mamet's main message is that to create drama, you have to show rather than tell. He writes: "The job of the dramatist is to make the audience wonder what happens next: not to explain to them what just happened or to *suggest* to them what happens next." In other words, the writer's job is to write the stinkin' scene, not tell us what happens in the scene or what's going to happen.
He continues: "Any time two characters are talking about a third, the scene is a crock of sh*t. Any time any character is saying to another 'as you know' that is, telling another character what you, the writer, need the audience to know, the scene is a crock of sh*t."
This advice is especially apt for fantasy or sci-fi writing. In these genres (the former of which I am currently attempting as my master's thesis), a world exists that the writer knows inside and out, but the reader is discovering it for the first time. The temptation is almost irresistible to have characters over-explain the rules of their world in dialogue or narration, but doing so would kill the drama of the story.
It all comes down to this: trust that your readers are smart and have faith that they will trust you, the writer, to show them what they need to know in due time. And remember that writing does not reside in dialogue alone; stage directions and description can do as much showing as do the words spoken by the characters.
As Mamet says: "If you pretend the characters can't speak, and write a silent movie, you will be writing great drama."