What is meant by "game mechanics"?
There is a fantastic article on this topic by Miguel Sicart called Defining Game Mechanics, it is mostly related to Video Games, but the the same definitions can broadly be applied to Board Games as well.

Essentially Game Mechanics are actions that modify the game state, and it is this modification of the game state that results in gameplay.
While Game Mechanics can, and should, be defined by rules, they are not themselves rules.

In a more colloquial form, if you're changing anything in the game, whether physical or conceptual, you are implementing a Game Mechanic.