What is happening to my dogs skin, this is under her chin

My dogs has this sore under her chin. What can I do to help heal it as it seems to be spreading. Thank you

Hello!   I wonder how quickly this is spreading?  There are some superficial skin infections ('wet pyoderma', my boss used to say) that itch, ooze pus and spread outwards at phenomenal speeds.  These can often be treated with prescription antibiotic shampoos, so are worth showing to a vet.  There are also some examples of crusty skin cancers, skin wounds, sun or parasite damage that may look a little like this.  Particularly as the lesion appears to be expanding, a chat to the dogs' own vet would definitely be a good idea.  Ideally they will triage the case (that is, check how urgent it is) for you and hopefully book an appointment.