Directly above the surface of a liquid, there is a small amount of vapor that comes from liquid particles that gain enough energy to escape into the gas phase. The pressure exerted by this vapor is called vapor pressure.
Watch this video to learn how this phenomenon occurs and how the strength of intermolecular forces affects vapor pressure.
Source: Khan Academy, https://youtu.be/mwVWQDIbF3A
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.