Yarn overs are used to make spaces in a piece of work, often creating a lacy effect, and generally adding an extra stitch.
On a knit row, with yarn at back of work, bring the yarn forward between the two needles, then back over the right needle to the back of the work. Knit your next stitch, and you will see you have added another loop to your needle.
© Reprinted with permission from Craft Yarn Council of America's Certified Instructors Program