A slot is a narrow opening in something that can be used for a purpose. For example, a slot in a machine is the hole that you put coins into to make it work. You can also talk about a slot in a schedule or program, where you have an appointment at a specific time.
A pay line is a line crossing each reel of a slot machine that determines winning combinations. In the early days of slot machines, there were only a few pay lines available, but as technology advanced, manufacturers began to use microprocessors to assign different weights to each stop on each reel. This made it appear that a certain symbol appeared more often on a payline than it actually did, thus increasing the jackpot sizes and the number of possible combinations.
Some slots have special bonus rounds that award credits to the player. These may involve a physical device, like a second or third wheel designed specifically for the game, or a separate screen with an interactive element. These bonus rounds can be triggered by hitting specific combinations of symbols on the payline, or by a combination of symbols appearing on a special pattern on the reels.
Researchers have studied how players perceive wins and losses in slot machines. They found that players overestimate their wins when the slot machine makes a celebratory sound and when they play with sounds turned on, but are much closer to accurate when the sounds are off.