Regroup means to re-organize your forces or to rendezvous. Usually used in an effort to create a collaborative attack. Example as follows:
You hit a bee hive with a baseball bat, then charge at the somewhat angry bees coming out. Swinging wildly into the air you hit about 50 of them. You think your doing quite well. But then they regroup. They sting the hell out of you, in a timely and extremely painful manner. This is the power of regrouping.