A DC-to-DC converter is a device that accepts a DC input voltage and produces a DC output voltage. Typically the output produced is at a different voltage level than the input. In addition, DC-to-DC converters are used to provide noise isolation.
This is were the voltage is uped to what ever the HU uses, probably like 24v or 30v instead of the 12v your battery puts out. It's done out side the HU to help prevent noise.