High/Low converters are used to convert a high level signal to a low level, in other words to connect an external amplifier to the main unit that does not have RCA outputs. Most modern amplifiers will allow connection to the factory head unit via the high-level input, but higher quality hi-low converters (especially active hi/low converters) represent a significantly better solution. And this is especially the case when connecting a subwoofer, because passive high/low converters have higher distortion and cannot work with the lowest frequencies.