Yarn overs are the simplest increases (they're barely even a stitch!) and, in our opinion, one of the cutest. Basically, they're an extra wrap around your needle in between stitches, that, when knitted later, turn into a little hole in your work.
Note that when you come back to a yarn over the row or round after you make one, the yarn over stitch will be very loose and open and not look like your other stitches. That's okay! Just knit or purl it normally.
If you haven't already, familiarize yourself with the different styles of knitting before continuing. There are three different ways to hold your yarn when knitting and you want to make sure you're watching the right technique for you.