A perk is something extra you get, in addition to a salary, in exchange for working. The major perk of your job at an ice cream shop might be all the hot fudge sundaes you can eat.
Some fairly common job perks include health insurance, free gym memberships, and company cars. You can also talk about the perks, or benefits, of a situation: "I have to ride to school with my annoying neighbor, but the perk is she always brings donuts." As a verb, perk means "to become more energetic or cheerful." So that donut perk? It might be just the thing to perk you up on a Monday morning.