Well, I've heard it called "bonking", sometimes I refer to it as "hitting the wall". Basically, the point where you don't have enough hydration/food to keep you going. This will happen to me if I go swimming after work and don't have some sort of food before hand: there's a point where I get cranky and my body just stops.

I've done a lot of reading on this, and basically it's making sure that if you're doing physical stuff that you have plenty of fluids (water and electrolytes) and some sort of snack/food that you can have either before or during, depending on how long you are being physical for.

For example, right before my swim last year, I made sure I had some breakfast before we got in the car, then I had a half liter or electrolyte drink before going in on the boat. I had one of those gel food packs on the boat, and I stuffed another gel food pack in my suit, which I had (most of) about halfway through.

It makes me wonder, in general, if people think that they can just eat the same as they had before doing a lot of physical activity. I tried to explain to someone once that I needed more carbs when I was swimming more than when I'm not swimming. She kinda looked at me funny (or maybe it was because of the fat thing? I don't know....)