Itterasshai means "Take care," "Have a good day," or "Please go and come back." This is the response given to someone who says "Itte kimasu." It's a warm send-off, wishing them a safe return. Parents often say this to their children as they leave. "気をつけてね、行ってらっしゃい！" (Be careful, have a good day!).

「行(い)ってきます」と
言(い)った
人(ひと)に
対(たい)して
言(い)う
言葉(ことば)です。
無事(ぶじ)に
帰(かえ)ってくることを
願(ねが)う
気持(きも)ちです。「行(い)ってらっしゃい！」