Help!

I'm not sure how this works, cause I wanna eat about 1800-2000 calories, then what do I do, when I workout? Cause mfp says that "*Net calories consumed = total calories consumed - exercise calories burned. So the more you exercise, the more you can eat!" I don't get, why I should eat more when I exercise, doesn't that make me gain weight? I'm confused.

If my goal is 2000 calories (average calorie intake for a woman)
should I eat 2500 and then burn 500 calories from my running? :-)