This is a casual and common way to say goodbye to someone you expect to see the next day. "また" means "again," and "明日" means "tomorrow." It's typically used among friends, classmates, or colleagues in an informal setting.

明日また会う人に使う、カジュアルな別れの挨拶です。「また」は「再び」、「明日」は「次の日」という意味です。