Something immediate is happening now or right away. If you're in immediate danger, you'd better run or call 911.
Immediate is a word that pretty much means "now." If you take immediate action, then there's no delay. If you need immediate assistance, you can't wait for assistance. The related word immediately should be a clue, since it also means "right now." If something is happening later, or it happened a long time ago, or you have to wait for it at all, then it's not immediate.