「海 (うみ)」 means "sea" or "ocean." It refers to the large body of saltwater that covers most of the Earth's surface. You use it when you talk about going to the beach, swimming, or enjoying the ocean view during holidays.

地球(ちきゅう)の大部分(だいぶぶん)を覆(おお)う大(おお)きな塩水(しおみず)です。夏(なつ)に海(うみ)へ行(い)きます。