colloquial phrase meaning "just so you know" or "I'm telling you" used to preface a remark (often a warning or reminder).
言っとくけど、明日は早く出発するから朝ごはんは自分で用意してね。
Just so you know, we're leaving early tomorrow, so please prepare your own breakfast.
言っとくけど、その映画はネタバレが多いよ。
I'm telling you, that movie has a lot of spoilers.
言っとくけど、本気で反対する人もいるから会議で発言するときは注意して。
FYI, some people strongly oppose it, so be careful when you speak at the meeting.