Molecules of a liquid have kinetic energy, and the kinetic energy of a particular molecule of a liquid constantly changes when it collides with other molecules. At a given moment, some molecules have relatively high energy and some have relatively low energy. Molecules with high kinetic energy can overcome the gravitational force of the surrounding molecules and escape from the liquid surface and enter the gas phase.
If the evaporating liquid is trapped in a closed container, the vapor molecules cannot move away from the liquid, and a number of vapor molecules return to the liquid phase. The molecule is equalized from the vapor phase to the liquid (condensate). The vapor pressure that equilibrates with its liquid at a certain temperature is called the vapor pressure.