What is the "Compressed client/server protocol"?

I googled around, and didn't find any information on it. What does this option actually do? Does it improve connection performance? Which servers support this kind of connection? Is it better to leave it on or leave it off?

There's a brief explanation at MySQL Internals Manual -> MySQL Client/Server Protocol -> Compression:

https://dev.mysql.com/doc/internals/en/compression.html

Since I have more spare CPU than network bandwidth I normally enable it. In practice, I've never had any issue with the feature.