purpose and usage of secondary-selection ???

[ken]

Sometimes while typing on my laptop I accidentally brush against the 
touchpad and inadvertantly mess up something.  Today, while not even 
emacs, this happened, affecting what happened in an emacs frame which 
previously didn't even have the focus.

What I ended up in that emacs frame was that a random section of text 
was set to background yellow.  Investigating further, I found that text 
with the yellow background was a secondary-selection.  But I couldn't 
find anything in the Help screens describing anything about it, except 
its properties.  So here are my questions:

What is/are the purpose(s) of a secondary-selection?

What can be done with a secondary-selection...? and how to do them?

How do I undo this secondary-selection... i.e., make the selection go 
away without changing the file/buffer in any way?


Thanks much.