yes, like the above poster said, a deficit means that you're burning more than you're eating.
a BMR calculator will tell you how many calories you burn just existing, like she said.
when you get that number, just make sure what you're eating is creating a deficit. for example, if your BMR is 2000 calories, and you eat 1500 (not counting exercise) your deficit is 500 calories/day and that will equal about a pound a week.
the numbers dont always come out perfect though, some weeks are just frustrating.
if you try to burn off more calories than you eat (i.e. working out to burn off 1400 calories when you're only eating 1300) your body will starve. your BMR is just enough calories to keep your heart beating, brain functioning, etc. if your total intake for the day was -100 calories, your body would start failing.
make sure the deficit isnt too much! never eat below your BMR (and dont count calories burned by exercise!)
not telling you what to do, just my personal opinion based on the medical advice ive gotten.