Atomic orbitals: 1s electron density
The "surface" of the orbital represents points for which the electron density for that orbital is the same  an isosurface. By choosing different values of electron density, denoted by the bar moving up and down on the line plot or by the moving plane on the surface plot, then the size of the threedimensional plot changes.
As the wave function is directionless, so is its square. All values are of necessity not negative since the square of any real number cannot be less than zero.
