Questions concerning the overall configuration of pawns in a game, e.g. what pawn structure features should be sought or avoided, when to alter the structure, how to work with one's pieces in a given structure, etc.
The pawn structure during a game is one of the more static aspects of a position, as the pawns are the least dynamic soldiers in the game and their moves are irreversible. In this sense the pawn structure gives a sort of skeleton around which the game is played; it shapes the battlefield where combat takes place. Knowing what sort of structures to seek (and which to avoid), as well as generally how to handle structures that commonly arise (e.g. when to change the structure with an advance or a capture), are thus important factors in one's chess abilities.