Button (Dealer)
A disc marking the nominal dealer. The player on the button acts last after the flop, which makes it the best seat.
In casinos a house dealer deals the cards, so the button shows who would be dealing. It moves one seat clockwise each hand, which rotates the blinds and keeps the game fair.
Acting last means seeing everyone else's decision first. That information edge is why strong players open many more hands from the button than from under the gun.