A slot is a narrow opening in something, such as a machine or a container. It can be used to put things into, such as a coin or a letter. It can also be used to refer to a schedule or program, such as a slot in the school day.
The simplest explanation of how a slot works is that each spin of the reels results in a random number being generated. This number is then paired with the symbols on the reels to determine whether or not you win.
Most modern slot machines use computer software to control the number of symbols on each reel and the random number generator (RNG) to generate a random number for every spin. This is a much more complicated process than it used to be, and there are often hundreds of combinations of symbols per reel. The chances of getting a certain symbol on any given spin are therefore much higher than they used to be.