Knit 1, Yarn Over, Knit 1 into Same Stitch (kyok)
The kyok -- or knit 1, yarn over, knit 1 into the same stitch -- is a double increase. It takes one stitch and turns it into three stitches.
To work it, knit into the first stitch on the left-hand needle without sliding it off. Work a yarn over around the right-hand needle as you normally would. Finally, knit once again into the first stitch on the left-hand needle and slide it off.
The following video demonstrates how to work a kyok: