This is a polite farewell said to someone who is leaving a place (like home or office) to go somewhere and will return later. It expresses well wishes for their journey or day, essentially meaning "please go and come back safely."

家や職場などから出かける人に対して使う丁寧な挨拶です。「行ってきてください」という気持ちを込めて、無事を祈ります。