First, your body fat percentage is simply the percentage of fat that your body contains. If you are 140 pounds and 20% fat, it means that your body consists of 28 pounds fat and 112 pounds lean body mass (bone, muscle, organ tissue, blood and everything else).

Men and women differ in their body fat levels according to their level of fitness and their age. Taking women first, the typical % body fat levels for female athletes is between 14 and 20%, fit young female adults range from 21-24%, for average women- acceptable levels would be from 25%-32% (under 40 years) and up to 35% in the over 40's.
And for men, male athletes would typically have levels of 6-13%, fit men would have levels ranging from 14-17%, acceptable levels for the average man would range from 18-25%.

N.B. The resting metabolic rate provides an estimate of how many calories your body needs a day while at rest, before taking into consideration any calories needed for exercise.