A crowd; a large number of people. Used as a noun (e.g., 'a lot of people').
コンサートには大勢のファンが集まった。
A large crowd of fans gathered at the concert.
大勢の前で話すのは緊張する。
I'm nervous about speaking in front of a large audience.
祭りの日は商店街が大勢でにぎわう。
On festival days the shopping street is bustling with crowds.
As an adverbial use meaning 'in large numbers' or 'with many people' (e.g., 'in great numbers').
観光客は大勢訪れるので、早めに予約したほうがいい。
Because many tourists visit, it's better to make reservations early.
大勢で押しかけるのは迷惑になることがある。
Showing up en masse can sometimes be a nuisance.