For public health & infectious disease specialists, the tale of "Typhoid Mary" is legendary. Mary, a cook in New York, had typhoid and over several decades managed to infect at least 50 people, 3 of whom died. The real number infected by her is not known with certainty.
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What "Typhoid Mary" illustrates is that not all infected individuals are equal, some are better spreaders than others, i.e. "superspreaders".

This phenomenon occurs in both human and animal populations, for various viral and bacterial infections.

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What's been observed has been described as the “80/20” rule: A phenomenon where ~ 80% of infections are thought to have been spread by 20% of infected individuals or “super-spreaders”.

It's been seen for a whole range of infections such as tuberculosis, HIV, SARS and MERS.
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For example, in Singapore back in 2003, 5 people are believed to have caused more than half of the 205 cases there.

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And similarly, in South Korea in 2015, 3 cases of MERS CoV infection led to 166 other infections. (Notable that healthcare settings were a high risk setting then too and many infected were healthcare workers)

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Why such individuals become super-spreaders is less clear.
Reasons include immune system deficiencies,
greater virulence of the pathogen, co-infection with another pathogen, delays in diagnosing infected persons, overcrowding, high risk settings like care homes...
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Why is this relevant now? Simply put, COVID19 demonstrates superspreading phenomenon.

Current infectious disease-control measures such as mass screening, contact tracing and isolation of infected individuals tends to be laborious and not always effective.
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If superspreaders can be identified early in an outbreak, may allow for more effective disease control. I.e., if the 20% of infected persons who spread 80% of infections are identified early & managed well, 80% of onward infections could be avoided. Priority research topic!
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I wrote (a not very good piece) about superspreading a few years ago:
https://theconversation.com/one-in-five-super-spreaders-responsible-for-the-majority-of-viral-infections-34832

But here's a better recent paper by Althouse et al:
https://covid.idmod.org/data/Stochasticity_heterogeneity_transmission_dynamics_SARS-CoV-2.pdf

We can't identify superspreaders so we need to continue to maintain physical distancing & hygiene measures.
Pre-covid I told my students there was little evidence for #facemask use. Been convinced by @trishgreenhalgh & will advise otherwise now based on precautionary principle.