A focus fabric is your the "main" fabric.. If you pick a fabric with cats, for example,that would be your focus fabric. Then you would find fabrics that coordinate (not always match, but coordinate) with your cat fabric. Any fabric that you use as a "focus" for your quilt is your focus fabric.
Here's a good link that talks about focus fabric and how to coordinate it:
http://www.mccallsquilting.com/mccal...a_Focus_Fabric