This is, as you can see, a jug. The (childish) scrawl on the glaze makes the shape of a oak leaf. Or several oak leaves. These crop up all over the country on sites of the Medieval period. Most of them were made for one specific pub: The Oak (or Oak Leaf, or Royal Oak, I'm afraid I can't exactly remember). This pub was famous for pilgrims, and of course, Canterbury was famous for pilgrims.
But then how did they get everywhere? Well, you'd go to Canterbury, and you'd be thirsty when you got there. So, you'd drop into the pub, buy your jug of ale and say to yourself "I'm having that!" Then, when you got home, you'd display it in pride of place so everyone would know you'd been to Canterbury on pilgrimage. And that you were hard enough to have nicked the jar.