The button is the position of the dealer in a poker game. While you are not necessarily dealing the cards yourself since it is done by a professional dealer in the casino or cards get distributed automatically in online poker games, that represents the actual seat at the table where the dealing starts.
The button position moves clockwise one seat with every hand so that every player can act as the dealer once per orbit.
For example, if you’re on the button pre-flop in Texas Hold’em, you’re in the most advantageous position out of all players. You’ll have more information than any other player when making your decision, so you can accurately determine ranges and adjust your strategy, and you always get to act last in all betting rounds after the flop.