In addition to manny's info, the ridge indicates the original thickness and the rotor has worn behind it, so to speak. You have to measure the thickness of the rotor in the middle and see whether it is above the minimum or better yet service level. On my 126, they come with 28 mm new, the service level is 25.8 (meaning they get replaced at that level when you have the car in for service) and the minimum is 25.4 if memory serves me right.
Mine had quite a ridge and I measured 25.9.
Good luck
Reinhard Kreutzer