Keep an eye on the hole and make sure your cat isnt licking it or paying a large amount of attention to it. Licking can cause the wound to become larger, more swollen and even cause infections.
Like Savas said above, it should heal over and go away in a week or so, but if it persists, swells, turns an unhealthy color like a deep red, brown or even a green, then it has gained an infection and a vet visit is something to be done.
Dont cover the wound up or put Vasaline on it as this will prevent the healing process. The more air the wound gets, the faster it will heal and scab over.
It's an abscess drain hole. Generally, these things take care of themselves, opening and closing and draining over a period of days or a week or two. If it's persistent and gets swollen or painful, you should get an antibiotic for it.