When you have to line up two pieces of material to create a corner, your pieces may be either flush, shy or proud. The illustration below shows what I mean:
A piece is flush when it lines up with the outside face of the other piece. A piece is shy when it is inset from the outside face, and proud when it sticks out a bit.
While your goal is to be flush, it is often better to be a little proud. A proud piece can be trimmed or sanded so it is flush. If you try to line your pieces up exactly flush, you may end up shy. When you’re shy, it is harder to make it flush after the glue has dried; you either have to use filler (which is often too fragile to hold a good corner) or sand the long piece of material on its entire face.
So, in fabrication at least, it is better to be proud than shy.