A circuit that will not pass a DC level or offset from its input to its output. Most audio processors and amplifiers have AC coupled outputs; they will not pass DC levels, and they usually filter subsonic frequencies from the output also. One reason for this is to prevent speaker voice coil burnout; if a DC offset is applied to a DC coupled amplifier, a large current will flow through the speaker coil without producing any audible sound, so there is no indication of a problem until the smoke appears.
However, synth circuits that are to be used to process control voltages must be DC coupled. Some VCAs and synth mixer circuits have switches that select either AC or DC coupled operation, so that the circuit can be used for both audio and control voltage processing.