A subfifth is an interval too narrow to sound like a perfect fifth and too wide to sound like a tritone. It is a peculiar interval in that it has a range of producing a massive amount of overtones to the point of heavily distorting the timbre of any instrument that it is played on. The interval is normally considered a 16/11 and is quite harsh regardless of timbre, but some are worse than others. Regardless of what the name suggests, a subfifth isn't really a type of perfect fifth but rather can be thought of as related to the tritone, however, the interval really is a class in it's own right.
The subfifth has a xen appeal because it is a sound that is so far from anything possible in 12edo or 5-limit just intonation that it resembles the very essence of the term "out of tune". Anyone who does not enjoy rough intervals will hate this interval range with a passion. This is for those who can find beauty in the harsh and dissonant.