While animal experts have not yet determined the exact signal sent by one dog licking another dog on the back, these experts have determined that when one dog licks another dog the message is that the dog is showing either submission or affection. Dogs will also lick their owners to show affection or to show submission as well.
There are instances when a lick could signal that the dog has a need such as food, water or to go outside. These licks can be directed at other dogs or at owners. Affection is one of the biggest reasons a dog licks because there are endorphins that are released in dogs when they lick people or other animals. These endorphins are the same endorphins that humans feel when they do things such as bite their nails or listen to music. The licking helps to release stress and give the dog a feeling of comfort with their owner or with another dog.
Submission is important in pack animals because it helps to keep the dogs at peace with one another. Young puppies will lick their mother's mouth and will lick the other dominant dogs in the group when in the wild. When at dog parks or when visiting with other dogs, a dog might lick another dog to signal submission and let the other dog know that it should not feel threatened in any way.