What is a whip stitch stitch?

A whip stitch in crochet means to thread a yarn needle with yarn and "sew" essentially. For instance, if you make an applique such as a truck, and wish to put it onto a blanket, you would thread your yarn needle with matching yarn, begin underneath, and stitch it to the blanket as you would sew a patch onto a pair of jeans. It also means to sew two peices together-as in if you have two straight edges like two granny squares to make an afghan, to stitch the two together.