Our bodies are made up of functional units, such as organs (brain, heart, liver, kidney), muscle mass, fat mass, extracellular fluid, and supporting structures (connective tissue, bone).
The fat mass is made up of fat cells and it acts as the reservoir of energy. Fats exist in these cells as triglycerides, which is made up of a glycerol molecule and three fatty acid chains, organized like the letter E, with glycerol forming the vertical line and the fatty acid chains forming each arm of the E.
When we consume else calories than our body needs for maintenance or by exercising to spend more calories, it triggers our body to spend energy to make up for the deficit by spending its energy reserves stored in the fat cells. To release the energy stored in the fat cells, hormones act as the messengers to start disassembling triglycerides into their components glycerol and fatty acids.
These components are then released into the bloodstream. The liver preferentially absorbs the glycerol and some of the fatty acids while the remainder is absorbed by the muscle mass.
After the triglyceride ingredients are absorbed into the liver and the muscles mass, they are further broken down into compounds called acetyl-CoA. The acetyl-CoA is then used up by the mitochondria in the cells and combined with the compound oxaloacetate to form citric acid and release heat alongwith byproducts like carbon dioxide, water, and Adenosine Triphosphate (ATP). ATP is the energy-carrying molecule that fuels cellular activities. Therefore, the fat cells are converted to useful energy that fuels your body's demands placed on it through exercise and other activities.