In the diagram below, particles of the substance are moving from the liquid phase to the gas phase at the same rate as they move from the gas phase to the liquid phase.

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the gas and liquid are
~ equilibrium.
~ a high vapor pressure.
~ a low vapor pressure.
~ zero vapor pressure.

The answer is equilibrium.

This is what is called dynamic equilibrium. It is the most common form of equilibrium in chemistry. It means that in average the system is not changing but if you look particle by particle they are changing. Giving that the rate at which the parciles pass from phase A to phase B is the same at which the particle pass from phase B to phase B, the system does not change, i.e. it is in equilibrium.