Going by what's been described, I can say it could be a big deal or just a minor one.
Does the cyst effect the dog in any way? If so, go to a vet and see about getting it removed.
If not, is the dog old? If the dog is elderly, and not effected by it, skip it.
My friends toy poodle had a fist size cyst on her side. It didn't effect the dog one bit. It died 7 years later of VERY old age. (Actually, they had to put her down. It was blind and going completely deaf.)