What do you understand by vapour pressure of solids?

Solids also evaporate and thereby exhibit a vapour pressure. The molecules of a crystalline solid operate with respect to their equilibrium position and are in a position to collide constantly with the nearest neighbours which lead to a distribution of kinetic energies. A small fraction of molecules at the surface of the solid possesses enough kinetic energy to break away from the surface. As a result, the free molecules enter the vapour state. The process in which solid is directly converted to its vapour state is called sublimation. This accounts for the vapour pressure of solids.