Resource guarding is defensive aggression that is limited to guarding food, water, toys, or other objects that are within the "ownership zone" of under a canine's mouth and between the front legs. It is not to be confused with "first right" to a resource (associated with dominance aggression). The rules drastically change once there is ownership of a resource. This is explained more in this video if you need clarification: Dog Culture Made Simple
It is important to understand that although resource guarding is a common occurrence with dogs that are prone to dominance aggression, it can and often is displayed by dogs as isolated behavior regardless of social status.
In fact, a study of natural wolf behavior by David Mech has quantified that resource guarding is a natural behavior of all wild wolves and has no correlation with status. A low ranking puppy is just as likely to guard food in the ownership zone as a pack "alpha" from any other pack member.
This type of aggression is not viewed as any form of status challenge and does not typically trigger any kind of retaliation from higher ranking canines.
This is important to note since this type of behavior should never be interpreted as a "challenge for status" or confirmation that a dog believes he has a dominant status over a person or another dog.
Some dogs naturally have a stronger instinct for this primitive behavior while other dogs have a less intense instinct due largely to selective breeding.
Understanding what the behavior is and what it is not, is an important first step to improving a resource guarding "problem".