Discoid is just like a bit of manufacturing defect in your knee. The cartilage which is normally supposed to be half-moon shaped has remained full moon. One may live a normal life with discoids without ever knowing about them, but once it is torn, as it is prone to that, it starts damaging the knee.
By arthroscopy, which is a pretty safe surgery, we give the cartilage its original shape - the half moon. It can be done on both knees at the same time. Two weeks of rest is needed if done in both knees together. Leaving discoids untreated is not good for the life of your knee.