The metabolic rate of the human body is basically the rate at which it consumes the energy it extracts from nutrients.
This energy is consumed for two basic purposes:
1) to maintain the body’s natural functions
2) to exert physical activity
The more muscle one has the more energy (calories) he/she will consume not only during physical exercise but when at rest also.
The body’s at rest metabolic rate is directly proportional with the lean muscle one has. (the lean body mass, which is basically all the muscle minus the fat)
Metabolic rate is subject to a few other factors as well besides the all determining lean bodymass. Metabolic rate varies with age ( it tends to slow down a bit as one progresses in age) gender ( men usually have more alert MRs than women), the functioning of the thyroid gland, etc.