In a home game of poker it is customary for each player to take it in turns to deal. The deal will pass around the table clockwise. The player to the left of the dealer will always act first once the hand has been dealt. By continually changing the dealer it will ensure that every player has to act first (a disadvantage) or last (an advantage).
If playing with a dedicated dealer, i.e. someone who isn't playing and will deal all the hands, it is necessary to keep these advantages the same. Therefore, it is important to play with a dealer button. This plastic disc is placed in front of a player who is the nominated "dealer".
Even though this player will not physically deal the cards, the non-playing dealer will deal the first card of the hand to the left of the dealer button. The player to the left of the dealer button acts first. After each hand, the dealer button is passed to the left and that player is now the nominated dealer.