Bruiser is a gear perk that improves your defense against melee attacks. It helps when soldiers rush you or you fight up close.
Enemy punches, rifle butts and blade attacks hurt you less in close fights.
Wear one of the gear pieces that carry it: Clown Body, Kheshig Armor, Protest Vest. Each piece only takes one body slot, so plan around the other perks you want.
Situational. Useful for melee builds, but most damage in Far Cry 6 comes from guns.