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?