Knee MRI shows large multiseptated lobulated synovial cyst at tibiofibular joint. What does this usually mean? Surgery? Could it be cancer?

Benign. This is a benign cyst; that does not become cancer. If it is causing symptoms it can be aspirated under ultrasound guidance or removed surgically. However typically it is am incidental finding on MRI and can be observed.
Baker`s cyst. No, baker`s cyst is an extension of the synovial cavity and membrane from the knee joint. If it is large and causing pain, decrease knee range of motion, difficulty walking surgical removal helps.