Why would i have subchondral bone edema in my knee? is this serious and what can be done?

This is usually caused by inflammation which has caused additional damage to the bone below the knee. I assume you have osteoarthritis of that joint and the MRI ruled out infection & injury. Anti-inflammatory meds like Advil, cold packs, physical therapy, using cane or crutch until knee symptoms improve should help but make sure to be treated & followed by an orthopedic surgeon.