TDEE (total daily energy expenditure) is the full number of calories your body uses in a day. Most of it — typically the largest share — is your resting metabolism, the energy needed just to stay alive; the rest comes from digesting food, ordinary daily movement, and deliberate exercise. A calorie deficit means eating below your TDEE, which is what drives weight loss.
On a GLP-1 the number matters in two directions: it's the starting point for setting a calorie and protein target when appetite is low (see what a calorie deficit is and the TDEE Calculator), and it falls as you lose weight — both because a smaller body burns less and because of metabolic adaptation. Calculator estimates get less reliable after large weight loss, so treat TDEE as an estimate to check against what the scale actually does (see do TDEE calculators still work after big weight loss).