Every direction you can face from a point on the surface z = f(x, y) comes with its own slope.
That slope is the directional derivative for the unit vector u pointing that way, and the continuum of directions means infinitely many such slopes exist. Find any one with Dᵤf = ∇f · u.
Hikers' GPS apps compute these rates to estimate energy cost when a trail turns in any compass direction over varying terrain.