The shaded regions are those which are most likely to be occupied by electrons. The probabilities vary as a function of the type of atom and its configuration. The electrons move in a region with a radius of the order of 1 angstrom, around a positively charged nucleus about 10000 times smaller. The zone in which electrons move defines the radius of the atom.
Only quantum mechanics can explain this model.
The 'Electron Cloud' button allows you to keep the past positions of the electrons on the screen.