The opening that grabs attention. The reason someone stops scrolling.
A hook is the first thing someone sees or hears. On social it's the first line. In a video it's the first three seconds. In an email it's the subject line plus preview text. A hook works by creating a pattern-break (unexpected), curiosity ('I need to know more'), or identification ('that's me'). Without a hook, nothing else you say matters.
In a world of endless scrolling, hooks are the entire game for the first three seconds. The best content with a weak hook gets seen by nobody. Bad content with a strong hook goes viral. The hook writes the check; the rest of the content cashes it.