To start many card games, it is part of the rules (or part of the tradition) for the dealer to shuffle the deck, and for the dealer's opponent (or one of the dealer's opponents) to cut the deck.
The deck is placed, face-down, in front of the cutter. The cutter lifts up part of the deck and puts it to one side. The dealer then picks up the bottom part that the cutter had left behind, puts is on top of the new pile, then picks up the whole deck and starts dealing.
The theory is, even if the dealer manages to surreptitiously arrange the deck to give him/her a favourable hand, the opportunity of cutting gives the opponent(s) a chance to rearrange the deck so the favourable cards are as likely to go to any player as to the dealer. And, since the deck is face-down, the cutter cannot see precisely where to cut to gain an advantage for him/herself. Cutting the deck is a very simple operation, with little chance for the dealer to manipulate the deck between the cut and the deal, when the dealer does it right. Most dealers will do it right, too, in order to demonstrate fair sportsmanship.
A player can decline to cut; to decline, the cutter gently knocks on the top of the deck instead of cutting.