What is "dog-earing"

Usage examples of "dog-earing".

Buckman pondered, meanwhile mindlessly dog-earing the edge of the Xerox page.

After dog-earing one smudgy photograph of a 250-year-old house and a handful of tourist guides, he'd been infatuated enough to follow his impulse out here.

I confess to marking my place promiscuously, sometimes splaying, sometimes committing the even more grievous sin of dog-earing the page.