What you burn doing nothing, what you burn on a normal day, and what to eat to lose.
Every TDEE number is an estimate, including this one. Mifflin–St Jeor is a population average fitted to height, weight, age and sex, so it can't see your muscle mass, your thyroid, your medication or how much you fidget, and individuals land roughly ±10% either side of it routinely. Treat it as a starting point to test against the scale over a few weeks, not a measurement. It is not medical or dietary advice, and if you're on a GLP-1 your calorie target is a conversation to have with your prescriber.
Your BMR drops every pound you lose, so a TDEE you worked out in month one is quietly wrong by month four. Tirzing recalculates it off your latest weigh-in and sets it against what you actually ate and burned that day: food diary, workouts, energy balance, all in one place.