that's how the wheels are measured in size to the car,and if that's the bolt pattern you have then that's what you need to ask for,if you give them the year and make of the car they will check the book to be sure they give you the right ones for it,here's a web site that will give the the right wheel size for that car look under your make and model and then compare their wheels to the ones on this site,it will tell you if yours are 5x100,good luck.
http://tirerack.com