CFL

I'm working at a University where we are developing and running interface software for CFD simmulations. There are no CFD specialists in my lab, however, so we get the information we need from a couple of other labs around campus.

Unfortunately, no one seems to know what the CFL number is, or how to use it.

Can I just get a simple explanation of the CFL number and a general range in which it should lie?

Thank you!