My right knee pains incessantly.a lump was seen in the x-ray behind the knee cap & doctor has suggested an operation to remove it. what exactly is it?

See details: Have you had an MRI done? An xray is not reliable enough to decide anything. It is impossible to give a definite answer without being able to examine an MRI first hand. Several thing may cause swelling.

Not sure: Unclear as to what lump would be- typically cartilage damage can occur in this area. i suggest asking specifically for doc to give you more specific info before having surgery