A payline is a fixed path across the reels that the game checks for winning combinations. Matching symbols usually need to land on consecutive reels from the leftmost reel along an active line to pay.
The earliest machines had a single line straight across the middle. Video slots added lines that zigzag through different rows; 10, 20, 25 and 50-line games are common. The help screen shows a diagram of each line.
On some older games you chose how many lines to play, and your total bet was the line bet multiplied by the number of lines. Most modern games fix all lines as active. Line wins are paid as a multiple of the line bet, so on a 20-line game a symbol paying 10x the line bet returns half your total stake.
Many recent games drop paylines in favour of ways to win, cluster pays or pay anywhere. See our paylines feature page for layouts and examples.