When knitting for the first few times, you'll definitely bump into this issue of holes forming at the sides of your sock's heels, otherwise known as pig's eyes. This usually occurs when you try to transition from rows to rounds when knitting and picking up stitches after turning the heel. However, the best way to prevent this is to pick up an extra stitch or two in the gap and then knit it together with the next stitch to close the hole. Though doing this is effective, you'll need to be careful because it might create a larger hole if done wrong. Check out this video tutorial to teach you how to resolve this issue.