A baker's dozen, also known as Long dozen, is 13, one more than a proper dozen. It may be rooted in the tidy way 13 disks (cookies, biscuits, etc) can pack a rectangle (baking tray) of appropriate proportions. It may also originate from the practice of baking 13 items for an intended dozen to prevent short measure. This was on the basis that one of the 13 would be lost, ruined or burnt in some way, leaving the baker with the original dozen.
Another explanation dates back to ancient times, where bakers — who, aside from selling bread, also baked the dough of housewives — were punished if they were discovered to be cheating by selling less bread (bread was sold by weight) or by pinching dough for their own use (often with the help of a boy and a trap door on the counter). As such, to prevent the crude punishment of getting their hands chopped off, bakers always gave 13 for the price of 12 to be certain of not being known as cheats.