Puppy discovers the mirror for the first time
This is Milo's first time experiencing a mirror and we can interpret his reaction in two ways. Earlier that day he had a reunion with his Golden Retriever brother Ollie. Milo may have thought Ollie came over to visit him so he was excited to play.
Most likely, this was his natural reaction to seeing "something" in the mirror. He tried to outsmart the other puppy in the mirror but became perplexed when the mirror-puppy mimicked his every move.
Either way it's a precious clip!