sewing slip stitch

Sewing question. What does it mean by slip stitch?
Im making a vest for fashion class. HElP PLZZ :)!!
http://www.geocities.com/diycat/resources/slipstitch.html
(Email me and I'll send you instructions for doing the entire vest, lining and all, by machine. No handwork except sewing on the buttons.)
How to Make a Sewing Tote Bag : Doing Slip Stitching for a Homemade Tote Bag