BMR - Basal Metabolic Rate
That means the number of calories you will burn if you stay full day in bed, in other words when your body is at rest. However this does not mean you sleep. You should be awake if correct BMR is to be calculated. So you can say calories needed in daytime for you to sleep in bed with eyes open can help you find BMR
BMR - Also means calories needed to maintain vital biological functions (kidney, lungs, heart, brain, pancreas etc) when at rest in day time
BMR is different for males and females.
Adult man may take 1600 calories per day
Whereas adult woman takes 1400 calories per day
BMR formula considers variables of height,weight,age and gender to calculate BMR.
RMR - Resting Metabolic rate
LBM - Lean body mass