http://www.mini-lathe.com/Mini_lathe...n.htm#Glossary
give that site a read, it has a bunch of basic terms that should help you get started.
basically what he was trying to say was that if you have your lathe in front of you and try to mount a bunch of different size of stock in there eventually you will hit the bottom of your lathe. So if you measure from the middle of your chuck to the bed of the lathe you will get the radius of the largest diameter part you can turn before you hit the lathe bed.
I actually like the description jhowelb gave...