Answer:
Osgood Schlatter's Disease is when there is an inflammation between the tibia and the patellar tendon, just below the knee.
It can cause a painful lump under the knee which could need to be operated on to remove.
It affects sporty kids mainly, but it is possible for it to occur in non-sporty kids (such as myself). The disease normally goes away by the time you're 16. It's mostly boys who are affected, but girls are as well.
It makes the knee hurt a lot when a person is doing physical exercise or walking, and stops hurting when a person is resting.
(I literally have it myself)