What is BMR?
The rate at which one's body burns through energy (calories) void of any activity, essentially the energy cost of the vital life functions is the Basal Metabolic Rate (BMR). Things like breathing, blood circulation, body temperature regulation, nervous system operations, etc., (staying alive) but not the extra calories burned during any additional physical activity such as getting swole or getting laid. Basically, if you just sat on your fat ass all day and didn't move at all, then you would need your BMR in stay alive.
What about if i don't sit on my ass all day?
That's what the calorie calculator is used for, accounting for your activity level to calculate the amount of calories you burn in one day. These are your maintenance calories and from here you can either subtract to go into a deficit, add for a surplus or alternate like on a recomposition plan. Depending on your goals and training you may increase activity level, gain muscle and burn more, so these numbers are dynamic throughout your program. The calorie counting system isn't perfect, a calories isn't always a calorie, a lot of factors go into this process but it's a good guideline and builds the habit of tracking variables.