Pain under the knee cap is usually caused by issues with the underlying cartilage (inflammation/ degeneration), or occasionally due to issues with internal knee structures, neuro-muscular issues etc. Without a clinical evaluation it would be difficult to comment on the possible cause here. You doctor might be referring to a knee clean up, where degraded structures and/ or inflammatory residue is removed, that helps in better and faster healing. It would be best to discuss the details with your treating doctor.
Hope this helps.
Thankyou yes your reply has helped.my doc did think it could be degeneration of the cartilage and I'm just waiting to be referred to a specialist thankyou again Dr jail.