BSL Physics Glossary - specific latent heat of vaporisation - definition
Look at the formula:
Eh = m x l
Eh means energy, equals mass times l.
l is the specific latent heat, either of fusion or vaporisation.
As you heat a liquid such as water, and put a lot of energy into it some of the molecules will change from liquid to gas. The bonds between the molecules in water will break apart and that will free them to become a gas and to jump out of the liquid into the air.
If we put in the same amount of heat all the time, we notice the temperature of the water rises. Just as it changes from a liquid to a gas, the temperature stays the same for a while. Then the temperature rises again. During that period when there is no temperature rise, the energy input is being used to break the bonds and change the material from a liquid to a gas.