Likewise, there are usually too many pieces on the board for theoretical positions to be completely analyzed as can be done in the simpler endgames.
To gain mobility is to ensure that the pieces have a wide scope of action and targets to focus on.
The concept is largely strategic in nature, and involves such concepts as space, pawn weaknesses (since weak pawns can compel pieces to defensive duties, reducing their mobility), and securing outposts for the pieces.
Material considerations are often secondary to pursuing the attack, and it can even be advantageous to lose pawns in front of the enemy king in order to open up lines for the rooks and queen.
Both players need to be on the lookout for pawn breaks, and the possibility of taking advantage of the open files which may arise from them.
For example, trading queens even at the cost of a ruined pawn structure may be a viable option.
Since many endgames involve the promotion of a pawn, it is usually good to keep that in mind when making trades during the middlegame.