It's a common way to mark a particular page for easy reference later. The term is often used as a verb as well, meaning to fold down the corners of a page.
Here are some examples:
* "I dog-eared the page with the recipe I wanted to try."
* "The book was full of dog-eared pages, indicating it had been well-read."
The term "dog ears" is thought to have originated from the way dogs often fold down their ears when listening attentively.