Are you talking about the YO? To YO on the knit side you bring the yarn forward between the needles. Then you'll put back over the right needle and knit the next stitch normally. It's supposed to leave a hole or eyelet.
To purl you wrap the yarn over the top of the needle from the right and around.
Here's a little more help. Sometimes it's easier to look at a few different explanations and then it will click. The first link has a good picture.