In “bdjw” (不懂就问), “就” functions as the conditional sentence structure “如果……就” (If …, … will …).
So 不懂就问 is actually （如果我）不懂，（我立马）就问。
If I don’t get it, I will ask right away.
That is to say, I will never feel shy or hesitant to ask if I have doubts.
So “bdjw” is a phrase used at the beginning when you’re asking something on the Internet, usually in forums or comments.