1A crowd, or a large number of people; used as a noun, as in 大勢の人 ("a crowd 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 get nervous 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 bustles with crowds of people.
2Used adverbially, as in 大勢で or 大勢いる, to describe many people acting or being present together.
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 a reservation early.
大勢で押しかけるのは迷惑になることがある。
Showing up in large numbers can sometimes be a nuisance.
家族や友人が大勢集まって誕生日を祝った。
Family and friends gathered in large numbers to celebrate the birthday.