Yes, it is a complete play on words. Everybody has heard nutritionists say "You are what you eat." The statement denotes that if you EAT garbage you ARE garbage because it makes you unhealthy and overweight. Students especially, should think of books the same way. If your literary diet consists more of comic books, graphic novels, young adult fiction, etc. then you are definitely not getting all that you need to become a more proficient reader and critic.
A good way to tell if you're reading too much junk, is to ask yourself this simple question: Are you embarrassed to tell your friends what you're reading? If so, it's probably junk. A good literary diet can consist of contemporary literature, classical novels, Early poetry, and assorted news articles. These things should really be the basis of what a student reads during their day.
Go ahead and indulge yourself, though. If you love Twilight or romance novels, knock yourself out! But remember to take time to read things to even out that indulgence so you don't get sloppy in your reading. When one reads only for entertainment, like if one only eats because it tastes too good to stop, then one is bound to lose control at some point.
For a well rounded diet I recommend reading:
Early English novels, such as Robinson Crusoe, Jane Eyre, Sense and Sensibility, Tom Jones and other works by authors of the time.
Early American literature including slave narratives, poetry pre-19th century, and women's literature.
For poetry, anything goes. Consider this the fruit of the literary world, it tastes good, and its good for you!
And don't forget short stories, and well known 19th century writers such as Mark Twain, Harriet Beecher Stowe, and others such as them.
There is so much variety, that even a "non-reader" can surely find something to enjoy, as well as something to increase their knowledge of history, and their ability to criticize literature.