I have avascular necrosis. What does this mean in simpler terms?

[Janders91]

This is what results of my knee X-rays said

FINDINGS:    
No acute fracture or joint dislocation is identified involving the left knee. Bones of the left knee do appear somewhat under mineralized relative to the right knee. Areas of sclerosis within the lateral tibial plateau and lateral femoral condyle are demonstrated, as well as within the distal femoral metaphysis. One or all of these sclerotic foci may represent sequelae of avascular necrosis given reported history. No significant joint space narrowing is identified. No suprapatellar joint effusion.


IMPRESSION:
1. No acute osseous abnormality.

2. Multiple sclerotic foci within the distal femur and proximal tibia, presumably representing foci of avascular necrosis given patient history. Follow up MRI may provide additional information, if indicated.

3. Generalized undermineralization of the left knee relative to the right, potentially related to disuse. Correlation with patient history in this regard recommended.