All of the responses given are correct, but the correct term for the "50," in your case, is called aspect ratio. This is a ratio of sidewall height to width.

If you multiply the width by the aspect ratio, you get the sidewall height of your tire. This is a good equation to use when you're figuring some ball park ideas on tire fitment.

Another thing, if you want to figure up the total outside diameter of a tire, you can take the sidewall height and multiply by 2. Then, add the diameter of your wheel. You probably won't ever need to do this, but at least you now know how.