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(あす)また会(あ)う人(ひと)に使(つか)う、カジュアルな別(わか)れの挨拶(あいさつ)です。「また」は「再び(ふたたび)」、「明日(あす)」は「次(つぎ)の日(ひ)」という意味(いみ)です。