Debugging is a methodical process of finding and reducing the number of bugs in a game, thus making it behave as expected.
Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge in another. Many books have been written about debugging, as it involves numerous aspects.
Game debugging tends to be particularly hard because of complex user behaviour (hard to reproduce), and structure of a program, i.e. the game loop, which runs constantly in "background" and thus constantly changes program state.