Basal metabolic rate is the amount of energy(expressed in calories) that a person needs to keep the body functioning, while at rest, in a neutrally temperate environment and in a postabsorptive state (meaning that the digestive system is inactive which requires about 12 hours of fasting).
In other words, if you just laid in bed all day doing nothing, then you would need to eat at least the number of calories roughly equal to your BMR to maintain your normal body functions.
This Basal Metabolic Rate (BMR) Calculator estimates your basal metabolic rate. If you are having a hard time calculating your macros and would like a professional to assist, click here.
Many people use this graphic in combination with out BMR calculator.