How is dynamic equilibrium related to vapor pressure?
When a liquid is in dynamic equilibrium with its vapor phase, the rates of evaporation and condensation are exactly equal to each other.
If you imagine putting a pure liquid into a closed container, the vapor pressure is initially close to zero, and as a result, the rate of condensation is also close to zero. The rate of evaporation is not zero, so the vapor pressure builds up to the point where the rates of evaporation and condensation are equal to each other. At this point, the system is in dynamic equilibrium, and the partial pressure of vapor in the container is equal to the equilibrium vapor pressure of the liquid.