Aim a projector upward at a wall and the image widens as it rises: a keystone, named for the arch stone. Correction pre-distorts the output so the projected result lands square.
Optical correction, lens shift, moves the image without cost. Digital correction warps the picture before projection, spending resolution to fix geometry: corner pixels get interpolated, fine lines soften. A little is invisible; aggressive correction visibly degrades.
The stage discipline is to solve geometry physically first (position, mount, lens shift) and let digital warping clean the remainder. Projection mapping generalizes the idea from fixing a rectangle to fitting any surface at all.