Video slot machines generally will only display the payline for lines that are winners.
A payline is a line (straight or zig-zagged) that crosses one symbol on each reel of a slot machine, or the combination of symbols on the slot machine reels that the player is paid out for if he has made a bet on that combination.
[1][2] Paytable, which is normally found inside the main game panel, gives information about all future wins and payouts.
Paylines, reward probabilities, winning combinations, and more can all be found here, as well as specifics about how to play the bonus rounds.
[3] Some machines offer symbols that are 'wild' and will pay if they are visible in any position, even if they are not on the payline.