Yarn Overs are used in knitting to increase the number of stitches and to make holes in the knitted fabric. Yarn overs are very useful in lace knitting.
Yarn overs are very easy. Like their name suggests, you just put the yarn over the needle. When I was first learning to knit, I was a little intimidated by them.
Step 1: Once you already have your knitting on the needles, knit to the desired point in your row and instead of sticking your needle into the next stitch, just wrap the yarn around the needle like you would when you are making a knit stitch.
Step 2: Knit the next stitch like you normally would.
Here is the Video, In it, I do a yarn over then a knit stitch, then another Yarn Over than another Knit Stitch.