An automatic needle threader is a miniature hook that goes through the eye of the needle from the back to the front, so that when the thread is placed properly in the hook, it will pull the thread back through the eye of the needle. Even if the end of your thread is frayed or if you have poor eyesight or bad lighting, an automatic needle threader will get your thread through the eye of the needle for you.
Be careful when using the automatic needle threader, because it can be damaged easily. The needle threader hook is wire-thin and can bend if the needle is not in its highest position and if the needle threader is not pulled down into the proper position. Learn how to use yours correctly by reading your sewing machine's user manual.
There are external automatic needle threaders that you can buy if your sewing machine did not come equipped with one. Some of them use a similar wire-thin hook to push or pull the thread through the eye of the needle.