Sight alignment is the geometry between your eye, the rear sight and the front sight - the target is not part of it yet. Correct alignment means the top edge of the front sight sits level with the top edge of the rear sight, and the light gaps on both sides of the front post are even. Shooters compress it into a formula: equal height, equal light.
A pistol's sight radius is short, so alignment errors are expensive: a tilt the eye barely notices grows into a miss downrange, and the error scales with distance. A front sight riding high sends the group high, a sunk one sends it low, and a post pressed to either wall of the notch pushes hits to that side.
Alignment is trained without ammo. In dry fire the classic exercise is the Wall Drill: working against a blank wall, assembling the same geometry click after click with the focus held on the front sight. Once it assembles automatically, alignment gets laid over a target and becomes a sight picture.