It could be a cyst which is generally harmless and doesnt bother the dog. It could be a small tumor that doesnt mean it is cancerous but needs to be looked at by a vet. You proabablly should have it checked out anyway. The vet can look at it and draw fluid from it to see what it is. Some times they may remove the place but if its not bothering the dog they may just leave it.
It can be fatty tissue if the dog seems sick with it or uncomfortable get the dog to the vet