It's a prepared tortoise food. It comes in two versions the original and the LS. Most torts only like the original which you can get online. The new version is called the LS and you can get it at petstores like petco petsmart or online. Some tortoises won't eat the LS though.
It's formulated food for tortoises to provide the nutrients they need, like fiber.
I feed it twice a week to all my tortoises. It's also great for picky eaters because, for example, my sulcatas and desert tortoise don't like to eat their Timothy hay on its own. So I rip up the hay and mix it in with the Mazuri (which is softened in warm water until it's a mush). My tortoises eat every bite!
Mazuri is the name of a company that makes balanced diets for many types of exotic animals, usually in an extruded pellet form. They make a diet for crocodilians, primates, ratites, tortoises of course, and many other animals. Most of us use Mazuri as a supplement to an otherwise good and varied diet to ensure that our tortoises are getting all the correct nutrients and in the right balance in case anything is lacking in the other foods we are feeding. It is also a convenient way to feed your tortoise when other foods are scarce, like during a frozen winter in some parts of the world, or in the fall here when summer has killed and dried up everything, but the winter rains haven't come yet to awaken all the wild weeds and grasses.
Thanks for the info and links, everyone! I actually was at Petco the other day and happened to spot some Mazuri Tortoise Diet on the shelves. I bought some and fed Archimedes some pellets later that night. He actually ate them dry/as-is, so I don't think I need to soak or mash them up.