A sign, painted on a clear glass door
, or posted on a sign. It is seen in many large office buildings
and the front of large department stores
It is an instruction to delivery men with large packages (generally, too big to carry under the arm). It means that the owners and/or renters of this building ask that you bring large deliveries to the back of the building, so as not to inconvience staff/customers.
It is not a sign that is displayed at gay bars. That would be crude, wouldn't it?