The kilocalorie per mole is a unit to measure an amount of energy per number of molecules, atoms, or other similar particles.
It is defined as one kilocalorie of energy (1000 thermochemical gram calories) per one mole of substance.
As typically measured, one kcal/mol represents a temperature increase of one degree Celsius in one liter of water (with a mass of 1 kg) resulting from the reaction of one mole of reagents.
This is due to a variety of factors, including the ease with which it can be calculated based on the units of measure typically employed in quantifying a chemical reaction, especially in aqueous solution.
In addition, for many important biological processes, thermodynamic changes are on a convenient order of magnitude when expressed in kcal/mol.