The dealer button (BTN) is used in poker to indicate the player who will be playing from the dealer position. It is the position at the table right before the blinds.
It is considered to be the best spot to be in as you will always act last in a hand, allowing you to exploit your position at the poker table. There will be plenty of blind stealing opportunities when you are “on the button”, and also many opportunities to win pots by bluffing post-flop. You can also extract more value when you expect to be ahead, since you have the advantage of position after the flop.