The button (dealer button, BTN)

The button is a small disc that moves one seat clockwise after each hand, marking which player is in the dealer's position for that deal. In home games this person physically deals the cards; in a casino or online a house dealer handles the cards, but the marker still tracks who is the nominal dealer and acts last. The small blind and big blind sit immediately to the button's left, so the button is the last non-blind seat to act before the flop.

It matters most after the flop, turn, and river: the player on the button acts last in those rounds, so they get to see what everyone else does before deciding. Preflop is a little different, since the blinds have already posted and act after the button, but on every later street the button has the final say. As play continues the button rotates so everyone takes a turn in each spot over time.

For a beginner, the simple takeaway is that this is the best seat in the game, so you can play more hands here than from earlier spots like under the gun. The cutoff, the seat directly to the button's right, is the next-best position, but the button beats it. Acting last gives you information, which makes choices like whether to value bet, continuation bet, or fold much easier. When people say they like to 'play in position,' this is the seat they mean.