Over 90% online video slots have five reels aligned in sequence one after another. A specific alignment of symbols in the reels is called a payline when it can result in a payout. It only results in a payout under two conditions – when the winning combination is on the payout table and when the player has activated the payline.
The number of paylines in video slot games can be anything between 5 and 1024 (the average number is between 9 and 50). Obviously more paylines means that more winning combinations exist, but it doesn’t necessarily have to mean larger average returns, because the number of paylines is not the only factor in this case. Other factors include the payout amounts in the payout table, the relative frequency of symbols on different reels, and whether or not bonus games are likely to offer payouts.
As the number of paylines grows, the other factors get adjusted, so that the average return stays at about 95%. More paylines doesn’t mean you’re going to to win more. The trend of adding more and more paylines exists only to make games more entertaining for players and more unique.