Itte kimasu means "I am leaving" or "I'm off," literally "I will go and come back." You say this when you leave your home or office, addressing those who remain. It implies you'll return. For example, when going to school, you'd say, "学校へ行ってきます！" (I'm off to school!).

家(いえ)や職場(しょくば)を出る(でる)時(とき)に、残る(のこる)人(ひと)に向けて(むけて)言う(いう)言葉(ことば)です。また戻る(もどる)事(こと)を伝え(つたえ)ます。「学校(がっこう)へ行って(いって)きます！」