So sometimes when you're finishing your knitting, the regular bind-off is just a little too tight and you need something stretchy. So instead of the regular bind-off you can work it this way. You're going to knit two stitches together, which gives you one and then you're going to put that one back on your left-hand needle and then you're going to knit two stitches together again.
Then you're going to put that one back on your left-hand needle, and continue on as such, knitting two together and slipping it back onto the left-hand needle, which essentially means you're knitting every stitch twice, which gives you a much stretchier bind-off. This is great for toe-up socks, where the bind-off needs to fit over your heal, and the necks of sweaters for people with big heads, like toddlers. So that is how to work a stretchy bind-off.