A "damp rating" means a fixture has been deemed suitable for a damp location.
A damp location is an exterior or interior location that is normally or periodically subject to condensation of moisture in, on or adjacent to the electrical components of a lighting fixture or ceiling fan.
Damp-rated fixtures should not be used in areas that are directly exposed to water whether it be rain, snow, or shower spray. Products that are damp-rated say “Suitable for Damp Locations” on their product page under Product Details.