An intransitive verb meaning to become crowded or packed (with people, vehicles, etc.). Commonly used for rush hours or busy places.
朝の電車はラッシュでとても混む。
The morning trains are very crowded during rush hour.
休日は観光地が混むので、早めに出発したほうがいい。
Tourist spots get crowded on holidays, so it's better to leave early.
このレストランは人気で夜はいつも混む。
This restaurant is popular and is always crowded at night.