What is 'desorption hysteresis'?
Hysteresis occurs when the state of the system depends on its past states---that is, the state of the system is memory-dependent.

The canonical example is a ferromagnet: if you apply and slowly ramp up an external magnetic field to a ferromagnet for some time, and then slowly ramp the magnetic field back down for the same amount of time, you might expect the ferromagnet to return to its original state, but it doesn't. Instead, if you track the magnetization of the magnet as a function of the applied magnetic field, you see these hysteresis loops. Each applied magnetic field corresponds, therefore, to two possible system states, and which state the system exists in will depend on its past history.

Desorption hysteresis is hysteresis applied not to magnetization but to adsorption and desorption, and your excerpt then goes on to describe the physical interactions that drive this phenomenon.