A hook is the main part of a song. And it’s usually the earworm of the track.
Most often, the hook has the most memorable melody of the song and lyrically expresses the main idea of the song. Because of this, the hook is often where the song title comes from.
A hook is usually a lyric of the chorus, either at the end or repeating throughout. Or it can be a standalone lyric, which is common in a Verse-Refrain song structure.
A hook can also be instrumental, even in a song with lyrics.