A pound is roughly equivalent to 3500 calories. Therefore, if you eat 3500 fewer calories than you burn per week (an average of 500 per day, seven days in a week), you will lose about a pound a week. 1000 calories fewer a day than you burn gives you about a two-pound loss per week.
What you burn is a combination of your baseline metabolism (what you burn just by being alive) plus all your activity. You set your general activity level when you set up your FitDay profile and that's what it uses to set your calories burned per day. Any exercise you do is calories burned above and beyond that.
When you say your calorie counter starts at 2900, are you referring to the calories burned/calories lifestyle bar? If so, that's what FD has calculated you burn in a day, based on the data you entered. If you take in 2000 calories per day, that should get you to your goal in the time you set as your goal date.
I hope that helps! Clear as mud, right