One of the primary purposes of any one nation's military is in the defense of one's own borders - shared borders complicate such defending.
Border Coverage is the complete distance of shared borders for a given nation. Borders are an important factor in forged national alliances and in managing incoming/outgoing traffic and commerce. In times of war, borders become increasingly important for the home guard / border patrol elements and longer-spanning borders are a natural hinderance for the defender (leading to a draining of material and finances). As such, this important value is included in the GFP ranking.
Values below displayed in Kilometers (km). Island-nations, such as Australia, are not penalized for lacking shared borders but do suffer from having to defend extensive coastlines.