Calories are units of energy. Another common unit of energy is the joule. We commonly use these in relation to food or our body's requirement, such as 2500-3000 calories are needed per day by an adult, etc. Where do we get this energy from?
Our body (and that of other organisms as well) obtains this energy through the process of cellular respiration. In this process, glucose is broken down into carbon dioxide and water and (most importantly) energy is also released. The chemical reaction for this process can be written as:
`C_6H_12O_6 + 6O_2 -> 6CO_2 + 6H_2O + ATP`
Here, ATP molecules (or adenosine triphosphate) are the energy molecules and provide us with energy.
When cellular respiration takes place aerobically (in the presence of oxygen), the energy yield (in terms of molecules of ATP per molecule of glucose) is much higher as compared to anaerobic respiration.
Thus, cellular respiration is the process which generates energy that our body uses and can be measured in units of energy such as calories or joules, etc.
Hope this helps.