help understanding calories, net, etc?!

MFP is set up so that your daily goal already has a deficit built in - meaning eat to goal every day, do zero exercise, and you'll lose weight. When you exercise and burn off more cals, you are creating too large a deficit, which can cause problems in the long run, stalling weight loss, leaving you hungry, tired, eventually exhausted, no energy for workouts, burn out, not to mention screwed up hormones and metabolism if you continue this way for too long.

This is why when you log exercise, the burned calories are added back into your goal - you are supposed to eat them back. Net calories are your calories eaten minus exercise cals. So if your goal is 1200, and you burn 300 in exercise, your net is only 900 calories for the day, which isn't much fuel for your body to function on, especially if you're exercising.

Food is fuel - eat your calories, and goal means goal - you want to get as close to that goal as possible to keep your body well fueled and functioning. Eating too little for too long, especially if you're exercising, gives your body a reason to store fat rather than burn it.

Does that help?