Since hockey socks are "footless" socks, meaning that the sock goes from ankle to knee, the socks can slip down the leg if not secured properly. The garter belt connects to the players socks with either clips or velcro. They are similar to suspenders and offer the same purpose!
The garter belt is a thin, black band that is worn around the player's waist. There are four clips that are hang down from the belt. One of them is connected to the front of the sock, and the other one is connected to the back of the sock, with same on each side.
The obvious pros of garter belts are to help keep the socks in place, and therefore, the shin pads are kept in place. However, some players prefer to not wear garter belts, as they think they are more hassle than help. If the clips come loose, the belts can irritate and distract the player during the game. It can be difficult to reattach the clips because of all the hockey uniform pieces that are worn on top of the belt, since the belt is connected to a piece of underwear.
Garter belts can be purchased from sporting goods stores and come in small, medium and large sizes.