A stretchy bind off in knitting is just what it sounds like: a method of binding (casting) off that gives the bound off edge some elasticity. There are several stretchy bind off methods to choose from, but this is one of the simplest to do (and remember) so I use it often. The result is quite stretchy, and makes a visible chain edge (like a regular bind off) on the right side. It's suitable for the edges of lace shawls when you want to be able to block thoroughly.
Knit 2 (just as you do to start a normal bind off).
Put the tip of the left needle through the fronts of the first two stitches on the right needle, from left to right.
Knit the two stitches together
Repeat steps 2 - 4 until you have one left on the right needle. Break yarn and pull through remaining stitch.