Reclaimed barn wood is wood that has been used in older buildings, like barns, built before 1930, which in earlier days were mostly constructed from timber. The wood from these barns is taken from these buildings, cleaned up thoroughly and then milled so that it finds use again. This reclaimed wood will appear rustic, will often have a patina that indicates its earlier usage.
In many cases, this barn wood will be of species that are not being used nowadays, like elm. The very fact that you are using wood that already exists, and have not cut trees for their timber, makes the use of such wood, a highly desirable and environmentally friendly act.
Reclaimed wood is durable and highly versatile. Floors made of this wood often show a character and rustic dimension that is almost impossible to match with newer wood.