Below explanation is taken from the wikipedia. BMR stands for "Basal Metabolic Rate" and is the amount of energy expressed in calories that a person needs to keep the body functioning at rest. Some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. Basal metabolic rate (BMR) affects the rate that a person burns calories and ultimately whether you maintain, gain, or lose weight. Your basal metabolic rate accounts for about 60 to 75% of the calories you burn every day. It is influenced by several factors.
There are many BMR formulas. See this link for the most important ones: https://en.wikipedia.org/wiki/Basal_metabolic_rate
The one we use on Track My Fitness is the "Katch-McArdle Formula":
This site gives you the exact amount of calories you need simply by providing your daily caliper measurement (to determine fat percentage). Remember though... this is not exact science and differs from individual to individual. So give it a try and adjust your numbers based on your body response. See how it works for you and adjust your calories to reach your goals. If you don't have an account, you can quickly check your BMR here in the following calculator.