It's the process by which gas molecules hit the liquid, lose kinetic energy, and get stuck in a liquid. Now, these two processes occur naturally at the surface of a liquid. Suppose I take a liquid and examine the surface of that liquid. What I will see is a line of molecules connected together by intermolecular bonds or non covalent bonds. Now, if I somehow increase the kinetic energy of these molecules, what will happen? Well, if one of these molecules gained enough kinetic energy, that energy will be able to overcome the energy due to the bonding.