From Brotato Wiki
Boxing Glove is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Boxing Glove increases your Knockback by +3. This can be a powerful, but niche pickup in the right situation where you are in great need for knockback.
For example, on Wave 11 The Enemy spawns. These will speed up until they hit you. But they are extra susceptible to being knocked back. So if you don't have enough damage output to kill them fast, and your weapons don't naturally have good knockback, Boxing Glove can be a powerful pickup to help you in that specific situation.