By “Swiss Cheese” they mean Emmental / Emmenthaler, right? There’s a bunch of Swiss cheeses, after all.

Anyway. Bacterial activity creating pockets of carbon dioxide. Those bacteria are already in the milk provided you didn’t pasteurise it, but if you did they can be added back (as a Propionibacterium culture), right before you add the rennet. It’s also one of the factors behind the flavour of the cheese.