According to Storied Salvage, a board foot is a measure of volume of a piece of wood. To calculate it, simply measure the length X width X height then divide that number by 144. There is even an online calculator to figure out board feet for you.
If you are working on projects involving reclaimed wood, Storied Salvage is a good page to follow on Facebook. I love their tagline "Architectural Salvage with Provenance."