The sentence, "It will rain today," means that there is a possibility that precipitation will take place in your area.Rain is a part of the water cycle.
1. Water is evaporated from lakes, oceans and other water bodies by heat from the sun.
2. The water vapours go up into the atmosphere and combine to form what we call clouds. This is called condensation.
3. When these clouds cannot hold any more water, they release the water vapour, which due to the low temperature, will turn into water. This is called precipitation. In cooler regions, it may occur in the form of snow.