In addition to classes, each HTML element can also have an id attribute.
There are several benefits to using id attributes, and you'll learn more about them once you start using jQuery.
id attributes should be unique. Browsers won't enforce this, but it is a widely agreed upon best practice. So please don't give more than one element the same id attribute.
Here's an example of how you give your h2 element the id of cat-photo-app: