Hockey slash guards are pieces of protective equipment that players wear on their wrists. They are often called wrist guards. Similar to the function of a shin guard in soccer, the guard protects a player's wrists from hits by another player or another player's hockey stick, just like shin guards protect against kicks to the shin.
Hockey is an aggressive contact sport. Players can "stick check" their defender, meaning that the player can try to steal possession of the puck by using their stick. In these situations, especially if the player is bending over trying to keep the puck in his possession, sticks can collide with players arms. Since the wrists are exposed in these situations, wrist guards help prevent bruising, breaks or cuts that can come from a "slashing" hockey stick.
Slash guards can be bought from sporting goods stores around the world. They range in colors, but many are black or white. There are different sizes you can try on to see what type of material feels most comfortable. The slash guards should not inhibit your arm movement or your game playing abilities, they are only there to take away an unnecessary injuries to your arms.