A "resource" is anything valuable or cherished.
Something protected for fear of losing it.
We all resource guard. If you lock your doors when you leave your house or car, you are resource guarding.
We lock up our house and its belongings to keep them safe. We keep our wallet close and conceal our pin number in the check-out line at the grocery store. We might even become violent if someone tries to harm something we really value, like our children.
Dogs also act to keep valued resources safe from theft. They naturally want to keep food and high value chews, bones, toys for themselves and become stressed if another person or animal tries to steal a prized possession.
If pushed, you might clutch your purse closer, call for help, or dial 911. Some people run away, some people fight back. Some carry guns.
Normal dogs passively ask, "Please don't touch." They pause and turn away. It's a polite request. Most will take the coveted item and move away to avoid conflict. (This can turn into a game of keep-away if pursued.)
Not all dogs who take items are resource guarding. Playful dogs use objects to invite others to play by parading by and then running to engage the housemate or person in a fun game of chase.
Like this dog:
Archie, the famous YouTube video dog thief, does what he does due to a combination of genetics and learning history. His compulsion to have things in his mouth is the "retriever" part of his heritage. His family thought it was cute and encouraged it. Through learning history and lots of practice, they have perfected his behavior of finding and presenting items, but not delivering them. It is a strong, resistant to change, generalized to many types of items, learned behavior. They could have taught him to clean the house if they'd added "put what you find in this basket!"
What do they guard?
Food, toys, bones, resting places. Some dogs are possessive of their people (fear of losing the commodity of attention) and will interrupt hugging and keep another family member from coming to bed if they are already sleeping with the other one. They may guard places like bedrooms or cupboards where food is kept.
Extreme cases guard where food or toys have been before but aren't now. Some guard water. For most the space bubble is a few feet from where they are currently enjoying the thing and only triggered if a person or another animal is clearly desiring their stuff. Others become paranoid, guarding much larger areas from people/animals who are actually paying no attention at all - they don't even know there is an issue.
Dogs who resource guard may:
eat faster or try to swallow a non-food item
take the item and move away
go still and give a side-eye stare or warning growl
escalate to snarl, air snap but no contact (a warning)
make brief contact, but little or no injury (a muzzle punch or nip)
If challenged, the dog may go belly up, or he may fight back
Some will take after any who come within quite a distance
worst case, a full-on crushing bite - or multiple bites
If history has proven that asking nicely is of no use, the dog who has been treated roughly in the past might go straight to biting. without warning
Unfortunately, we make resource guarding worse by scolding and chasing the dogs to get stolen items back.
Many cases of extreme resource guarding and ingestion of non-food items (swallowed socks, etc) is the result of chasing dogs down, wrenching the item out of their mouths and punishment which only destroys trust, creates avoidance, and escalates the matter. Don't follow recommendations from anyone who recommends that you "have to be the pack leader" or "show him who's alpha." Making a point of bothering a dog when it is eating or taking things away "because I should be able to whether he likes it or not" just creates an adversarial relationship. Now, whenever the dog sees you coming, he prepares for the worst.
Teach a Willing Trade!
One of the most sure-fire ways to increase resource guarding, keep-away and swallowing items is to chase the dog down and extract what he has from his mouth. DON'T!
Read this article by Pat Miller from Whole Dog Journal:
Positive dog training techniques for getting back items you don't want your dog to have.
January 24, 2017
Here is a great video on how to teach your dog to "drop" something he has.