The app lets students add photos to their assignments and share images and files from other apps. Google states:
Right from the assignment page in the mobile app, students can snap a photo and attach it to their assignment — whether it's the experiment they just did for a science class, or a drawing they made of their family tree. And if they've forgotten their homework, they can ask someone at home to snap a photo, text it and then turn it in with the app.
Students can also easily attach images, PDFs and web pages from other apps to their assignments. For example, when you're in a drawing app, you can create a graphic for an assignment. When you click "share" in that app, Classroom will come up as an option, and you can attach the graphic to an assignment.
The Classroom app also caches class streams and assignment information so they can be accessed even when the student is offline. It's available for anyone that has a Google for Education account.