One of the most common problems new knitters have is that their knitting doesn't stay the same width as they work because they are inadvertently either adding or subtracting stitches as they go.
A common way of accidentally adding stitches to a knitting project is by wrapping the yarn over the needle when you aren't forming a stitch.
It's not very attractive, however, when you aren't doing it on purpose. Yarn overs will also give you more stitches to work with, which could mess up your pattern and will certainly make your work progressively wider.
This commonly happens when switching between knits and purls in the same row. Remember to always move the yarn from the front to the back between the stitches, rather than by wrapping it around the needle.
Another way it can happen is if you take the working yarn over the needle as you begin a row of knitting, so that it looks like another stitch. Sometimes if you pull up on the working yarn at the end of the row you'll see the stitch from the row below will begin to look like a stitch you should knit, which will add a stitch without making such a dramatic hole.
How to Fix Holes in Knitting
There's only one way to remedy this situation, and that's to unknit (through procedures knitters lovingly refer to as tinking or frogging) beyond the hole and start knitting again from that point. Another option is to work to the stitch you made with the yarn over and drop it on purpose until it no longer exists, but this will cause loose stitches as the work tries to absorb that extra yarn, so ripping out is really the best course of action.
If you are just practicing and don't mind the holes or don't want to rip out, you can work a knit 2 together at some point in the row to get down to your original stitch count if you want. Or just leave it.
If accidental yarn overs are a consistent problem for you, count the number of stitches on your needle every few rows. If you’ve got more than you started with, odds are you slipped in a yarn over at some point.
Making Holes on Purpose
Holes can also be used for decorative purpose in knitting by playing with the gauge of a yarn--and another reason you might have holes is because you are using a larger needle than you should for the pattern. For instance, yarn can be knit on larger needles than normal to make an open fabric, like in the pattern for the Trendestter Capelet. This is a fun effect, and using yarn on different sized needles is a great way to learn about yarn.