Piece activity is one of the most important strategic factors. As we will later see piece activity is not limited to mobility, but also includes other aspects like piece coordination. For this lesson however we focus on the mobility of pieces.

The mobility of a piece is the number of squares to which it can move. You can simply count these squares, but keep in mind that control of the centre and generating threats on your opponent’s side of the board is of greater value. Depending on the chess piece this results in some other sub goals. 

Knights have to be centralized. There are several chess aphorisms referring to this principle: A knight on the rim is dim and A knight on the side cannot abide.

Bishops should not be locked behind the pawns of their own color. A Bishop which is hemmed in by pawns of its own color is called a bad Bishop.

Rooks are best placed on open files, but as you may notice the number of reachable squares doesn’t increase by centralizing them, while they become more vulnerable.

One of the goals of the opening is to develop your pieces: increasing their activity. Avoid moving a piece twice during the opening is also a good chess strategy originating from this activity goal. This means that when you have developed a piece, it should not be moved again until the other pieces have been developed.