What Is Subtalar Fusion And How Is It Done?

for the past 2 years i am having foot pain in the left leg, now i am having pain in my right foot it and my knee is also painful when i contact a ortho doctor he said that i am suffering fromsub talar fusion and x ray shows that i am having accessory navicular with arthritis fothe subtalr joint he adives me for the suggery

please give me what is sub talar fusion and how will they do?