From BR Bullpen
A defensive substitute is a substitute who replaces a fielder. For example, if the manager of a team leading in the ninth inning decides to replace a poor fielding outfielder with a better defender, the player brought into the game is a defensive substitute.
Very few statistics are kept concerning defensive substitutes, although a few players have had seasons where almost all of their playing time came in this capacity. For example, in 1978, Pepe Frias played in 73 games, including 61 at second base, while only collecting 15 at bats: he was almost always used as a defensive substitute for an aging Dave Cash in games in which the Montreal Expos were ahead in the late innings.