welcome home; welcome back
A fixed polite greeting said to someone who has just returned home. Often shortened to おかえり in casual speech.
"I'm home!" "Welcome home."
お帰りなさい、旅行はどうでしたか。
Welcome back! How was your trip?
From the honorific prefix お + the 連用形 (continuative form) of 帰る (to return) + the polite imperative なさい (from なさる). Literally 'please return', now a fixed greeting.