This is a long-used term that isn't specific to COVID-19, which describes a legitimate medical phenomenon with a wide historical basis. Ever head of Typhoid Mary? She was a superspreader.

Interestingly, people who claim everything about COVID-19 is a conspiracy by the media (or the elites, or whoever) to control the populace with fear are significantly more likely to be superspreaders. Why? Because all the simple, moderately-inconvenient precautions they refuse to take legitimately do slow the spread of the disease. Someone who frequently goes within 6 feet of other people while not wearing a mask and leaves their home more frequently than necessary has a much higher chance of getting the disease, a much higher chance of transmitting the disease to each person they come into contact with while contagious, and also a much higher number of contacts to potentially infect.

The term is actually incredibly useful, even on an individual level - if I'm considering who I'm willing to interact with, someone who's come into close contact with 500 people in the past week while not wearing a mask is significantly more dangerous to me and my loved ones than someone who has come into contact with 50 people at a distance of 6 feet or more while wearing a mask.