The two tigger striped kitties just said hi to each other and did a head bump. I think the bigger/older cat must be mom or dad. But given how aggressive it seems to be, I'm guessing dad.
That's the first time I've ever seen any of the cats do the traditional cat head bump.
If you have one or more cats, try doing a gentle head bump with them. Before that tho, you probably want to go nose to nose and sniff them. I've done "cold" head bumps with my cats but their domesticated so it's not like they could've shown me all of the nuances.