Refers to the end of the week, typically Saturday and Sunday. Can be used as a noun or adverbially.
週末はいつも家族と過ごします。
I always spend the weekend with my family.
週末に友達と映画を見に行く予定です。
I plan to go see a movie with a friend this weekend.
I'm going shopping with a friend on the weekend.
I spent a quiet weekend with my family.
Well then, have a nice weekend!