At it simplest, rain is described as drops of liquid water falling from the sky.
Clouds form when water vapour (a gas) cools and condenses to form water droplets (a liquid) but these droplets are so tiny they stay in the sky. To become rain, the water droplets must grow by acquiring more water and becoming larger. Once they become heavy enough, they can fall to Earth.
Some droplets collide with others in order to become larger whilst others will grow as water condenses out the air into the droplet. When these drops become too heavy to stay in the cloud, we get rain.
The size of raindrops is highly variable, from as small as 0.5mm in diameter to 6mm.