Shooting technical advice
It means that the wood is in contact with the metal in every place it should be, with no gaps.
This is particularly important at the head of the stock, which is where the recoil forces are transmitted from the metal of the action into the wood.
A sloppy fit in this region can over-stress the parts of the wood where the metal touches, and cracks can result.
Usually, a visual inspection is all that is required to discover whether the wood-to-metal fit is good or bad.
It is desirable to have the wood standing slightly proud of the metal. Then, if the wood gets slightly damaged, marks or very small chips can be sanded out without causing unsightly low spots.