Fretboard radius is ...I'll explain clumsily. Looking at the fretboard from end to end, it will be a convex surface. If you match the arc of that surface to a circle, the radius of that circle is the fretboard radius. Small numbers mean a more curved radius, like the Strat, which I think is 9.5". Gibsons--acoustic and elctric, are all mostly 12". Old Telecasters were about 7.5". Martins tend to be about 15" I think, amybe 17". The more curved(smaller number) fretboards are easily to make barre chords on, but harder to bend strings without interfering with the fretwires, flatter ones are more difficult to barre, but have less potential for buzzing.