If you are resentful of someone, you feel ill will toward him, mixed with envy, like when a newer coworker gets the promotion you've been working toward for years.
The verb resent comes from the word for "feeling," but we only use it to talk about bad, stinging feelings. You might resent having to work late, but if your boss pays you a higher wage for overtime, maybe you’ll be less resentful. When there are clear inequalities among people, it often makes them resentful. Someone who wears themselves out doing heavy labor might be resentful of their boss who never breaks a sweat but earns more money.