WOD stands for Width of Distribution. It is the number of retail stores that a product is distributed to - which then is also the number of stores that customers can buy the product from.
For example, if an product is available at 100,000 stores across the country - it's National WOD is 100,000 and if it's available at 10,000 stores in a city, it's city WOD is 10,000.