An active payline is a line across a slot’s reels that is included in the current bet. Winning combinations only count when they fall on a line that is switched on; identical symbols on an inactive line pay nothing.
Slots handle this in two ways. With fixed paylines, every line is always active and the total stake covers all of them. With adjustable paylines, the player decides how many lines to enable, which lowers the cost per spin but also cuts the number of ways a spin can pay. The Bet Min button on some titles switches lines off for exactly this reason.
On the operator side, clear line settings save support time. A player who disables lines and later sees a combination on one of them may think the game is faulty, so showing the active line count next to the stake avoids confusion.