The riddle
What Has a Head but No Brain?
What the head is for
Flat and broad, the top of a nail is called its head. It gives a hammer something broad to hit, and it stops the nail from sinking right through the board. A nail has a head, a shank and a point, and it has no brain, so every word of the riddle is true of it. A hammer's own head is the part that strikes the nail's head, so two heads meet in every blow.
Another answer that fits
A head of lettuce or cabbage is the other answer people commonly give. It has a head and no brain too. Other things fit as well, such as a pin with its pinhead, so the riddle is loosely worded and the nail is simply the answer used here.
What the d on a box of nails means
Nails are sold in "penny" sizes, written 8d, 16d and so on. The letter d stands for the old British penny, which was abbreviated d from the Roman denarius. The number points to length: a common 8d nail is 2 1/2 inches long and a 16d nail is 3 1/2 inches. The link between number and length is historical rather than a formula, so it has to be looked up on a chart.
Why riddles use heads
The word "head" is one of the busiest in English: the head of a table, a hammer, a river, a class. Riddle writers lean on it because almost every object has some part that can plausibly be called one.