When a fuel is burnt, a lot of heat is produced. Now let us understand why combustion of fuels results in liberation of heat or why fire is hot. When a chemical reaction occurs, rearrangement of bonds takes place. Certain old bonds are cleaved (broken) and certain new bonds are formed. Cleavage of bonds requires energy whereas during the formation of bonds energy is released. The amount of energy required breaking a bond or the amount of energy released during the formation of a bond depends upon the nature of atoms involved in the bond.
The reaction in which the energy released during the formation of new bonds is more than the energy required to break the old bonds are accompanied by the evolution of heat and are called exothermic reactions. On the other hand, reactions in which the energy released during the formation of new bonds is less than the energy required to break the old bonds are accompanied by the absorption of heat and called endothermic reactions.
The reaction involving combustion of fuels are highly exothermic because in case of combustion the energy released during the formation of bonds in the product of combustion (CO2 and H2O) is very large as compared to the energy required for breaking bonds in fuel molecules and oxygen molecules. This is why the combustion of fuel produces lot of heat and fire is hot.