To cram is to crowd or stuff people or things into a limited amount of space. You might be tempted to cram your mouth full of your mom's chocolate chip cookies, especially if they just came out of the oven and are still warm and gooey.
You can cram yourself with food, or cram your books onto a shelf that's not quite big enough to hold them. A writer might cram one book with dozens of characters, or you could cram homemade pancakes with blueberries. Another way to cram is to study for a test all in one session: "I haven't studied at all yet — I'll have to stay up tonight and cram." The Old English root, crammian, means "stuff something into something else."