In chess, "piece activity" refers to having pieces that are more "active," i.e. not blocked by other pieces or tied down defending other pieces, and therefore able to create threats and attacks against the opponent. In this sense, "piece activity" is a type of "positional advantage" in chess (as opposed to a "material advantage"). Even if two players have exactly equal material on the board, the player with more piece activity can often times be in a vastly stronger position and much more likely to win.