If it's on the underside of the foot pad, it's most likely a sore hock then. If the skin color is red and inflamed at all then it needs at minimum a change of flooring to softer less abrasive surfaces so it won't continue to irritate the sore, or it may be to the point it needs vet care. Or if there's a swollen lump or open wound it absolutely needs vet care as it means it's most likely infected and your rabbit needs an appropriate rabbit safe antibiotic(usually a minimum of 4 weeks, though sometimes much longer) and the anti inflammatory meloxicam.
I can't tell for sure, but in the photo it looks higher up on the leg and not on the foot? If it's not on the underside of the foot pad but higher on the leg, then most likely it will be an abscess or possibly a growth of some sort, both which should be checked by a vet and the appropriate meds and treatment given.