I came across this idea
here. The
effective size of a given population is, from what I understand, the minimum idealized number of individuals you would need from that population to reproduce the genetic diversity of the
actual population. It's not easy to find an explanation of this stuff in layman's terms, but
this article at genomena.com seems relatively understandable. Actually, here is one in
Scientific American that goes over all this stuff, and very briefly.