”새해 복 많이 받으세요” is the honorific way of saying “Happy new year” in Korean. This way of saying “Happy new year” is the most common.
To review the honorific ending and when to use it, see my lesson on 세요.
To wish people close to you a happy new year, you can use the casual form above.
받으세요 and 받아 are conjugations of the verb 받다, meaning “to receive.”