They're related but not the same, and confusing them is the most common reason calorie targets go wrong. Here's the difference in one line, then the detail.
BMR is what you burn at complete rest. TDEE is BMR plus everything else you do — walking, working, exercising, even digesting. TDEE is always the larger number, and it's the one you use for calorie goals.
Someone calculates a BMR of 1,600, decides that's their "eating number", and eats 1,600 to lose weight. But their TDEE might be 2,300. Eating 1,600 isn't a sensible deficit — it's a 700-calorie hole that's hard to sustain. BMR is a component of your target, not the target itself.
As you lose weight, both BMR and TDEE fall, because a smaller body costs less energy. That's why a fixed calorie target eventually stops working. The planner recomputes both every week so your deficit stays intact.
TDEE. It reflects your real daily burn, so a deficit from TDEE produces predictable loss. BMR alone ignores all your activity.
It shouldn't be for any sustained period. This site caps weight-loss targets at your BMR to keep them safe and sustainable.
A lighter body burns fewer calories at rest and in motion, so both BMR and TDEE decrease. Recalculating periodically keeps your plan accurate.