ball screw lead...

Ugh... I almost hate to ask, becuase it exposes my lack of knowledge...
but here it goes.

As a disclaimer, I've tried to find what it means via google, but ball screw
and lead are such common terms, it seams, that I can't find a page with an
explanation.

What does the 'lead' value of a ball screw mean?

Thanks says the newbie.