It's a blister, put a bandaid on it but do NOT pop it. Blisters heal on their own and the fluid inside them helps heal it. If it pops on it's own or you pop it, that's fine but it's best to just leave it unpopped if possible.

Yeah that's a blister. Put a band-aid over it. Eventually it'll pop (you can do it yourself if you want like with a sterile needle) and then it'll heal up. Band-aid is to keep it clean and prevent it from hurting you too much when wearing shoes/walking.