I'm 37 yrs old, bad knee pain, knee MRI says osteochondral lesion & subchondral cyst marrow edema suprapatellar effusion. What's wrong with my knee?

Inflammation. Your knee is inflamed. See a doctor. Consider lifestyle changes to address underlying inflammation. Try anti-inflammatory foods; avoid inflammatory food. Eliminate toxic exposures. Exercise judiciously--isometric exercises to begin. Progress to passive, then active, range of motion exercises, strengthening and endurance exercises. Read "Foods That Fight Pain" by Neal Barnard, MD.