DC Coupling (sometimes also known as AC-Blocking) is an electronic engineering arrangement that allows both AC (eg. audio) and DC (eg. control voltages) to pass into or out of an amplifier or other circuit.
AC-coupled equipment effectively has high-pass filters at each connection, and these affect the phase response slightly, so some designers prefer to omit those and provide DC-coupled connections instead. The danger, though, is that any DC present on the connections can potentially cause damage and incorrect operation of connected equipment, as well as loud clicks or pops when making or breaking connections. An increasing mumber of audio interfaces now provide DC-coupled inputs and outputs to enable them to be used with synthesizer control voltages, which are effectively DC signals.