It's commonly used in meteorology to describe a type of cloud with a heaped, puffy appearance, like a cotton ball.
For example, "cumulus" is part of the names of different types of clouds:
* Cumulus humilis: Small, low-lying, fair-weather cumulus clouds.
* Cumulus congestus: Towering cumulus clouds that can develop into thunderstorms.
* Cumulonimbus: Thunderstorm clouds with a towering, anvil-shaped top.
So, when you see "-cumulus" in a word, you can think of it as referring to something piled up or rounded like a heap.