At some (unspecified) time; someday; sometime in the future or at some point in the past (not specified).
いつか日本に行きたい。
I want to go to Japan someday.
彼はいつか戻ってくるだろう。
He will return someday.
いつかこの問題の答えがわかる時が来ると思う。
I think a time will come when I'll understand the answer to this problem.
この写真はいつか撮ったものだ。
This photo was taken sometime.