It`s the corner ,where if the guitar is laying on your bench,the face of the fretboard meets the side of your neck,from the nut down to the body,both sides of the guitar neck.Not face and back.You will be rounding the EDGES of the fretboard(It would be all trim getting rounded if you have a border).and the ENDS of each fret.Not the wood on the face inbetween.
Think like the edge of a countertop or table.
Your quote above sounds to me like you are describing scalloping.
There is a tutoral somewhere on how to do`The rolled edges `(Like Wes described) that edges your frets at the same time.It`s like an angled block that holds a file that puts a uniform bevel on all the frets.Then the sanding sponge.I believe it is on the Jemsite.I also use that method.
I did it to a OLP Stingray copy and used that bass for several years.It fealt awesome.
(Sorry if my description is confusing