Cumulus clouds are clouds that are piled up on top of each other. "Cumulus" means a pile in Latin, as in our word "accumulate" meaning to pile up. They are puffy clouds. Sometimes cumulus clouds are white and there are only a few of them in the sky; other times they can be heavy and gray.
Usually you get cumulus clouds when warm air rises from the ground carrying water vapor with it by evaporation. When the warm wet air rises up and meets some cold air, the water vapor gets colder and condenses into drops of water, making a cloud.
More cumulus clouds
When cumulus clouds are white and puffy, that usually means it's not going to rain right away. But if they grow into big gray clouds, you can expect rain.