Resource guarding isn't a hard behavior to understand. Think of the positive benefits of guarding resources for our dog friend's ancestors. As a dog you have worked hard to earn your food or to make a kill and the last thing you want is for some other animal to come and take it from you.
resource guarding is an instinctual behavior in dogs but not one all dogs display because not all dogs have had an reason to need to guard resources. If you have raised your dog since he was a puppy and he has been the only dog and there was never any competition for his food or toys then he has no need to guard them. If, however, your dog is a rescue that spent a good bit of time living on the streets then he or she may very well have lots of reason to guard his food bowl and toys.
As resource guarding is such an easy behavior to understand it probably doesn't need to be restated that a resource guarder is not a bad dog but it can be slightly unnerving when a dog owner goes to pick up a food bowl and gets an aggressive response or when a dog becomes attached to a toy and won't let it go or leave his side and any action towards that toy is treated as a threat by the dog or when a dog becomes attached to one person in the house and will guard them against any advances. While the behavior is one of the easiest to understand it is also one that can pose the biggest risk to the owners and our natural reaction when met with aggression is to lash out with aggression that can be in the form of yelling or worse for a dog and this will only make the problem worse.
We once did pet sitting and dog walking for a border collie that was a resource guarder. He was not allowed to possess anything and had to perform tricks in order to earn his food. Eating to him became a game and he understood that his resources where not his until they were given to him by his owner or whoever was feeding him. Another way to deal with a resource guarder is to give them a little bit of food at a time in a bowl different from the one they were guarding at varying times during the day. That way they learn that when an owner is coming to their food bowl it isn't to take a resource away but to give more and there is no competition for the food.
Resource guarding of toys can be a more difficult behavior to modify but is no different than any other behavior modification as it has to be down step by step. The first step is to remove the toys that the dog is guarding. Then using a toy they aren't guarding or a new toy that doesn't have value to them teach them the give or drop command by exchanging a high valued treat like a liver chew for the toy. Once they are done eating the treat give them back the toy with a command like take it and always praise them for dropping or giving the toy back to you. Work on this exercise with toys of ever increasing value to your dog until it is time to give them back their favorite toy.
Resource guarding is one of the most natural behaviors a dog can display. In a state of nature resources are limited and our dog's instinctual brains are going to function in that manner. As with most of the behavioral issues we see in dogs resource guarding is only an issue because of our coexistence with dogs in human society.