Condensation is the opposite of evaporation. It takes place when water vapor in the air condenses from a gas, back into a liquid form, and leaves the atmosphere, returning to the surface of the Earth.
Usually in order for condensation to take place, the atmosphere must be fully saturated. In other words, the maximum vapor pressure must have been reached.
In addition to being saturated, the nature of water requires that there be a surface upon which water can condense. This surface might be blades of grass or windows. In the atmosphere, condensation often takes place around dust particles or other particulates such as smoke and even microscopic bacteria.