A symbol is any picture that can land on a slot's reels. Wins come from landing enough matching symbols in the positions the game counts, whether along a payline, across ways or in a cluster.
Symbols fall into a few groups. Regular paying symbols are split into low-paying and high-paying sets. Special symbols do jobs: a wild substitutes for others, a scatter pays or triggers features wherever it lands, and bonus symbols start feature rounds. Many modern games add cash-value symbols, mystery symbols or collector symbols.
On fruit machines the classic set was cherries, lemons, oranges, plums, bells and bars, and those images survive in classic slots today. The Liberty Bell used horseshoes, card suits and a bell.
What each symbol pays, and what it does, is set out in the paytable.