Perhaps you've just finished a big meal at a restaurant when the waiter brings out a little dessert: "On the house," he says. Perhaps you've bought a new computer, and they throw in some free software and a discount on your next purchase. Or perhaps you've been to a bakery, ordered a dozen bagels and received 13.
Those are examples of a lagniappe: a little gift you get for buying something. It's an especially common practice in certain corners of the world, including Louisiana, where the term originates.