8-bit characters -- what in the world are these?

[Benjamin Sher]

Dear friends:

Under Composition in Thunderbird, there is the following option:

"For messages that contain 8-bit characters, use quotable printed MIME 
encoding".

Having no idea what this is all about, I decided to do a little research 
in Google. I found a lot of articles, read several including an article 
in Wikipedia, which you would think would explain it in plain English. 
Well, it does not. Neither do most of the others I checked. And I don't 
think it's fair to expect the user to go through thousands of articles 
to get a simple, practical answer.

What, in plain English, is all of this 8-bit character stuff about? And 
what are its PRACTICAL implications? What does it mean to us in real 
life in our email programs? Why should we check or not check this option?

Would really appreciate an answer.

Thank you.

Benjamin