Zenithal priming primes a model black, then sprays white (or light gray) from directly above — the zenith. The result is a monochrome lighting map baked into the primer: bright up top, shadowed underneath.
Thin paints and speed paints applied over it inherit that lighting for free, which is why the technique anchors so much modern miniature painting. An airbrush gives the cleanest gradient; a rattlecan angled overhead gets most of the way there.
See it in context in our paints & primers guides, or explore the tools hands-on in The Bench.