Net calories are the calories you eat minus the calories you burn. As long as your net is in the negative, you're going to lose weight. A general rule of thumb is that if you average a net of -500 calories a day, you'll lose a pound a week; -1000 a day to lose two pounds. If you're not severely overweight, 2 lbs/week is generally considered to be the highest possible rate of sustainable weight loss.
Your net calories depend on a lot on how many calories you burn. So, for example, there's a base number of calories that you burn just by being alive, and this is altered somewhat by what your lifestyle is like (sedentary vs. active, or somewhere in between). FitDay calculates this number based on your stats. On top of this, though, you can exercise to burn extra calories. You'll input this into the Activities Log, and FitDay subtracts that from your net calories, as well.
So, for example:
Yesterday, I ate 1463 calories.
Before I put my activities in, FitDay claims that I burn 2266 calories a day. So, if I hadn't exercised, my net calories would have been -837.
After I've put in my activities for yesterday (sleeping*, running, weight lifting, and circuit training), it says that I burned 2434 calories, so my net for yesterday was
I hope that helped! And feel free to ask any other questions, there are plenty of people around the forums who are very familiar with this technology! Welcome to FitDay!
*A note about sleeping. In theory, FitDay estimates 8 hours of sleep a day and incorporates that into your "lifestyle" calories. In practice, most people here seem to like adding sleep as an activity--it brings down the total number of calories that FitDay claims you burn, which then tends to line up better with estimates available elsewhere.