In chess, an increment is an amount of time automatically added to a player’s clock after each move, preventing games from ending purely on time pressure.
For example, in a game played at 10 minutes + 5 seconds increment, each player receives 5 extra seconds on their clock the moment they press the clock button after making their move. A player who moves quickly can actually gain time over the course of the game, while a slower player will see their clock gradually decrease.
In practice, the increment fundamentally changes time management: it becomes very difficult to win "on the clock" against an opponent whose moves come naturally and quickly. This encourages higher-quality play and reduces the frustrating scenario where a winning position is lost simply because the clock runs out.