Once you’ve set up your Fitbit and added your weight, age or gender (or all of the above), Fitbit calculates your BMR (Basal Metabolic Rate). This is what you burn just by breathing, digesting food, lying staring at the ceiling etc. It’s the only stat that will continue when you don’t have your Fitbit on. Even if you don’t wear your Fitbit, it will still say you burned these calories as you will have burned them no matter what, whether you spent the day lying on your sofa, or running a marathon, all without wearing your Fitbit. The absolute minimum calories burned would've been a definite.
(Although running a marathon without your Fitbit on would definitely suck as seeing the stats for that are great! 😎).
I hope this answered your question and welcome to the Fitbit community! 😊
