Particles with momentum p = mv are sent toward the slit one at a time, and you see where on the screen (plane A) each one lands. Change the parameters (on the right on a wide screen, below on a narrow one) to see how the spread of the landing points changes.
Horizontal axis = position x on the screen (plane A); vertical axis = |ψ(x)|2 (normalized to a peak value of 1; “probability in bin” when the distribution is extremely narrow).