Refers to the most memorable, climactic part of a song, often repeated. In J-pop and similar contexts, it typically corresponds to the chorus, but can also mean a catchy instrumental or vocal hook.
The intro ends, and finally the chorus kicks in.
The hook of this song is easy to remember right away.
Everyone sang along during the chorus.
Everyone sang the chorus loudly together.
This song really picks up energy at the chorus.
As soon as the chorus started, the audience all raised their hands.
メロ labels the verse or pre-chorus section of a song, whereas サビ specifically means the chorus or hook, so the two are easily mixed up when discussing song structure.
Katakana spelling is standard for this music term.
Hiragana spelling is also used, but katakana is more common.
From the noun 錆 (さび, 'rust'), possibly via the idea of a 'rusty' or rough vocal quality in traditional music, or from the verb 寂びる (さびる, 'to become mellow/patina'). The exact derivation is uncertain.