While some organs, like the brain, work continuously, other body tissues, like muscle and fat, can enter an inactive state. Muscles are typically inactive except during exercise, and a special type of fat — called brown fat — activates only in the cold to help warm us up.
When on standby, these two tissues burn very few calories and thus have a negligible impact on weight loss. But there's a notion that, if you build your muscles through exercise, those bigger muscles will burn more calories throughout the day. It follows that a person with a higher muscle-to-fat ratio would burn many more calories while at rest than someone with a lower ratio.
