What is resource guarding? No injuries. It is usually just mouth playing. The puppy is 10 lbs and the older one is 20
Mouth playing does not mean aggression exactly. Mouthing and making noise is normal play if both dogs are more or less equal participants, no one is scared and no one is hurt. No injuries often (but not always) means that the dogs are not serious about fighting. Best to keep an eye on them to learn more though. Do the dogs have some give and take in their interactions? The size difference isn't huge so that's helpful.
Here's an example of normal rough housing
[video]https://vimeo.com/185734485[/video]
[video]https://vimeo.com/223915928[/video]
Resource guarding is when a dog protects something he considers valuable by using aggression. For example, if there is food in the bowl and one dog is eating it while another approaches. The dog that is eating snaps or bites or attacks the other dog to protect/guard his food/resource. It can happen with food, toys and treats most often but some dogs will even get snippy over a couch or a bed space.