Nimbus clouds often mean that a thunder storm is brewing: there may soon be thunder and lightning. Get out of the swimming pool! Nimbus really just means a cloud that already has rain or snow falling out of it. Can you see the rain falling in the picture to the right?
So you often see the names of clouds combined: a cumulonimbus cloud is a cumulus cloud, a puffy thick cloud, with rain falling out of it, and a stratonimbus cloud is a stratus cloud, a gray blanket cloud, with rain falling out of it.