Hi/Lo converters are used to convert a high level signal to a low level, in other words to connect an external amplifier to a head unit that does not have RCA outputs. Most modern amplifiers will allow connection to the factory head unit via a high level input, but higher quality hi-low converters (especially active hi/low converters) are a much better solution. This is especially true when connecting a subwoofer, as passive hi/low converters have higher distortion and cannot handle the lowest frequencies.