What is a cumbulonimbus cloud?

Large raincloud.

There are two parts to this question. The word itself stands for two separate things. Cumulus clouds are the large clouds that often usher in thunderstorms or heavier rains. We also see them in some nicer weather days popping up in fair weather. Pilots call these cumulus clouds popcorn cumulus, or fair weather cumulus because no rain or bad weather is occurring at that moment. They can become bigger issues as the day wears on.

Nimbus is the second portion of this word. Nimbus means water is leaving the cloud and hitting the ground.

Therefore your cloud is a large puffing storm cloud that is emitting rain that is hitting the ground.