Book-to-bill is the ratio used by industries to measure their demand-to-supply ratio for their orders. It is the ratio of the number of orders received to the number of orders filled.
A company with a book-to-bill ratio larger than one means that it has more orders than it can fill at it's current capacity. A ratio equal to one means that it is able to meet all orders received. A ratio less than one means that there are less orders than it is capable of delivering.
The most prevalent usage of this monthly ratio is within technology and semiconductor companies.