RDM stands for Remote Device Management. This is the colloquial name for the ANSI E1.20 standard managed by the PLASA organisation.
RDM allows bidirectional communication over the DMX512 cable. This occurs on the twisted pair connected to pins 2 and 3. This same pair of wires is used to transmit data from the console to the dimmers or moving lamps.
RDM requires intelligent DMX splitters that can monitor the data for a command to reverse the direction of the cable.