Your basal metabolic rate (BMR) is the rate at which you burn calories at rest just to maintain vital body functions like breathing, heartbeat, and brain activity. Your BMR usually accounts for at least half of the calories you burn in a day and is estimated based on the physical data you entered when you set up your account: sex, age, height, and weight.
The calorie burn estimate that Fitbit provides takes into account your BMR, the activity recorded by your tracker, and any activities you log manually.
Your tracker's calorie count will reset each night at midnight and begin counting immediately thereafter. BMR is the reason your tracker starts the day with calories already burned—you've still burned calories even if you haven't gotten out of bed yet.
Note that in certain regions, you see kilojoules instead of calories in the Fitbit app. To change your units of measurement, see How do I change the units of measurement on my Fitbit device?