Piece activity refers to a piece’s ability to influence as many squares as possible, participate meaningfully in the game, and generate concrete threats on the board.
An active piece is well-placed: it controls important squares, disrupts the opponent’s plans, and can quickly shift to where the action is. A passive piece, by contrast, is pushed to the edge, blocked by its own pawns, or reduced to a purely defensive role. For instance, a knight on a3 or h3, far from the center, is typically passive; a knight firmly posted on d5, at the heart of the board, is a textbook active piece.
In practice, regularly assess each of your pieces individually: ask yourself what square it occupies, how many squares it controls, and whether it contributes to your current plan. If a piece fails on all these counts, look for a way to reactivate it — even at the cost of a tempo or two. Players in the 800–1400 ELO range often lose games not for lack of tactical skill, but because one or two pieces remained idle throughout the entire game.