DC, or direct current, is electrical current that flows in a constant direction. In a modular synthesizer, DC signals are used to control parameters through modulation. Audio is different: it is an alternating current (AC) signal that swings above and below zero volts, while DC signals are used mainly as control voltage (CV).
Control voltages are DC signals that set parameters such as pitch, filter cutoff, and amplitude. They do not produce sound directly. They control the modules that make or change sound. A DC signal can be steady, providing a fixed setting, or it can move slowly over time when shaped by envelopes, LFOs, or sequencers.
When unwanted DC ends up in an audio signal, it is called DC offset.
