A nimbus cloud is a cloud that produces precipitation. Usually the precipitation reaches the ground as rain, hail, snow, or sleet. Falling precipitation may evaporate as virga. Rain comes out of nimbus clouds and this is called precipitation. Usually formed less than 2000 feet. They bring continuous rain/snow.
Since nimbus clouds are dense with water, they appear darker than other clouds. Nimbus clouds are formed at low altitudes and are typically spread uniformly across the sky.