A BBS door is a mechanism to execute and communicate with an external program (usually a game, in which case it is also called a Door game) on bulletin board systems.
The BBS software starts the external program, and the door system passes data back and forth between the door program, the BBS, and the remote user. To supply the door program with the user's information (such as the user's alias and the amount of time they had spent online), the BBS software creates a dropfile containing information for the program to read. A few door programs including Front Line operated the opposite way, as Front Doors, answering the modem and then giving control to the BBS software. This method was particularly commonplace for hubs of networks.
Source: Wikipedia, BBS door , available under the CC-BY-SA License.