In the band diagram of the lower graph the origin of the x-axis is at the interface between the insulator on the left and the semiconductor. Further to the left is the metal gate. Within the semiconductor the band bending and densities should remind you of the p-region of the pn-junction. A positive potential has been applied to the gate which gives a field across the device which is shielded by the depletion of the p-type semiconductor.
If you increase the voltage applied to the gate should the depletion layer grow thicker or thinner? Try it by clicking the Vapplied slider in small increments.
Now try decreasing the applied voltage. The depletion layer becomes thinner until it disappears altogether at about -0.36 V. What happens if the gate is made even more negative? We might speak of the negative gate potential attracting holes to the insulator/semiconductor interface. The shielding of larger negative voltages applied to the gate is accomplished by attracting excess holes to the interface rather than driving them away as occurs for positive applied voltages. The layer with enhanced hole densities near the surface is called an accumulation layer.