What is the nucleolus?

The nucleolus is a region in the nucleus that contains RNA. See this dark spot in the onion cell? That is the nucleus, one of the few organelles you can see with an inexpensive microscope. If you look carefully at the nucleus you may see a dark spot. This is the nucleolus and it is found in eukaryotic cells.
What is the function of the nucleolus?
It has been described as the brain of the nucleus and contains RNA.
Transcript
http://www.moomoomathblog.com/....2020/08/what-is-nucl

Onion Cell Picture
https://www.flickr.com/photos/kaibara/3839720754