Energy is required to boil a liquid in order to give the molecules enough energy to overcome intermolecular attraction forces. While boiling, the highest energy liquid molecules are able to escape the liquid and enter the gas phase.
A substance can exist as a solid, liquid or gas. Solids are the lowest energy form, with molecules vibrating together and staying in contact. Liquids have more energy than solids, and while there are frequent particle collisions, they are able to slip by each other and flow. Gases have the most energy, with molecules being able to bound around and only infrequently colliding.