A thin opening or groove in something: a slot in the wall for a doorknob; a mail slot in a post office; an airline flight slot that allows for a certain amount of time for taking off and landing. Also: a slot in the schedule of work or play; a position, berth, or time for an event.
In a slot machine, the player pulls a handle to spin a reel or set of reels (typically three) that have pictures on them. If the pictures line up with a pay line, you win. The amount you win is determined by how many matching symbols you have along the pay line and the size of those symbols.
Modern mechanical machines have given way to electrical ones, but they still work the same basic way. A computer records a sequence of numbers, and the number at each position on the reel corresponds to where that symbol will stop. The computer then uses an internal sequence table to find out what symbols are on the reel, and which of them have a higher or lower probability of coming up.
Understanding how a slot works is one of the most important aspects of playing it. Having that knowledge can help you maximize your wins and minimize your losses. It will also make it easier to understand the payouts and bonus features of a slot game.