「海」 (umi) means "sea" or "ocean." You use it to talk about the large body of saltwater that covers most of the Earth's surface, or a specific part of it, like a beach. Many people enjoy going to the 海 for swimming or relaxing, especially in summer.

地球の大部分を覆う広い塩水のことです。泳ぎに行く場所です。