A kidney filters blood (thats where pee comes from), so if you get one removed, you might not filter enough, thus not urinating enough. That can be harmful to your body by keeping a bunch of unfiltered blood which was meant to be expelled in your body.
Many people live long lives with only one kidney. Many of those never even know they only have one until it is discovered on autopsy. Knowing help since you can avoid things as mentioned by the person above my response. Don't fret too much.