I feel like this is handled pretty well in the documentation.
prel: is relative to your player's coordinates on the grid. If you are at 10,10,10 and do something with prel:1,1,1 it will perform the action at 11,11,11.
vrel: is relative to your viewpoint in an F,Y,R scheme. Forward, Y, Right.
It's harder to explain in a simple way.
There's also hvrel (Horizontal view relative), crel (Cursor relative), trel (Target relative), tvrel (Target view relative), And then even a few more that I haven't used yet.