"Straight" refers to the fuselage, wing, etc being nice and straight, square, assembled correctly with no twists, bends, etc EXCEPT with those intended, as stated in the plans.
The fuselage should look nice straight down the keels, all the formers straight and true, even looking lines, stringers consistent in spacing, etc.
The wing should not have any warps, ribs spaced properly, leading edges conformed to the desired shape as per plans, and once again stringers true, and consistent.
Very seldom does a wing have no dehidrel (look like a V slightly, from a front or rear view), so leaving the wing flat in this case is a no-no.
Even the Spitfire, which looks like a fairly flat wing, has some dihidrel in it.
Once again, the plans will give you a profile view of how much dihidrel to put in.
The stab and rudder should be nice and flat, no warps or twists.
A "crooked" plane never flys properly, looks out of sorts, and can generally ruin your day when you try to trim it to fly and are not successful.