물다 is biting down and holding on, and the adverb rates it — 세게, in three lines, is biting hard, while 예의 있게 무는 편이에요 is a joke about biting politely. Two lines here come from a zombie game, where 이렇게 물면 전염이 되거든요, 'a bite like this passes it on'. The set phrase 물고 늘어지다, bite and hang on, means refusing to let a subject drop: 진짜 집요하게 물고 늘어져요.