The molecules of all substances have a constant disorderly movement. In addition; between molecules, there are forces of attraction whose value determines the state of aggregation of the substance. The Intermolecular forces of attraction are weaker in liquids and gases.
The molecules that are in the free surface of a liquid are subject to an intermolecular force weaker than that affecting molecules that are in the interior of the liquid. These molecules are attracted only by molecules that are below them in the liquid.
At the free surface of a liquid the velocities of the molecules may reach values such that their kinetic energy can overcome the intermolecular forces. So the molecules are able to leave the liquid and enter the gaseous phase.
The kinetic energy of the molecules of the liquid increases with increasing temperature. For this reason, the heating of a liquid accelerates the evaporation process.