w = griddata3(x,y,z,v,xi,yi,zi) fits a hypersurface of the form w=f(x,y,z) to the data in the (usually) nonuniformly spaced vectors (x, y, z, v). griddata3 interpolates this hypersurface at the points specified by (xi,yi,zi) to produce w. w is the same size as xi, yi, and zi.
(xi,yi,zi) is usually a uniform grid (as produced by meshgrid) and is where griddata3 gets its name.