What are the white things on outside of English Muffins?

These nooks are actually just pockets of air that form during the baking process. The dough is made with a high proportion of yeast, which produces carbon dioxide gas as it ferments. This gas gets trapped in the dough, creating these distinctive air pockets.

The nooks are what give English muffins their characteristic texture – they are chewy and slightly crispy on the outside thanks to these air pockets.