conjunction
if that's the case; then; if so
Colloquial conditional conjunction derived from the copula だ. Used to present a consequence or suggestion based on a preceding statement or situation.
See also: だ
明日は休み？だったら、映画に行かない？
You're off tomorrow? If so, want to go see a movie?
お金がないんだ。だったら、買わないほうがいいよ。
I don't have any money. Then you'd better not buy it.