When we do something, our body loses energy or burns calories, these calories come from the food we eat.
When we eat any food, first it get digested then the simple form of that food is absorbed into the body and produce calori.
But, even when we are not doing anything, our body still burns calories or loses energy, to control internal body functions.
Such as breathing, brain function, blood circulation by the heart and digestion of food absorption of food etc.
When we eat any food, there is a certain amount of calories that produced in the body from that food.
In the same way, there is a certain amount of calories burned to digest that food.
Now the ratio of the calories produced from a food and the calories burn in digesting that food is called Thermic effect of that food.
Now, foods that have a high thermic effect will require a lot of calories to digest, therefore, most of the calories generated from that food will be burned digesting that food.
So it is better to consume those foods which have higher thermic effect, that helps to maintain body weight and helps to keep the body healthy.