An indirect defense is when you do not protect a piece, instead you make it impossible for your opponent to capture it without taking a big risk. It may be hard to grasp so here is an example:
Many times your opponent will still blindly capture it and then he will lose something, maybe even the game. It is important to ask yourself:Why doesn't he protect it? Then you will see it. The indirect defense is a good tactic that instead of passively defending you do something else.
Hope you like it!