This is a big deal.  Once your characters start hitting that threshold, their effectiveness in combat drops pretty hard. If your endurance hits zero, not only does your character automatically have to catch their breath on their next action, but until they do and get endurance back into the positive range again, they are slightly more vulnerable to attacks.

Oh, and the same thing applies to enemies, too. You can see their endurance level too, and how quickly it drops based on their various attacks.  While there are a lot more factors in play, whichever side hits low endurance levels first is generally at a pretty distinct disadvantage.​