A supercell, according to Wikipedia, is a type of thunderstorm "characterized by the presence of a mesoctclone: a deep, persistently updraft." The above picture from here is a good example of a supercell. Other types of thunderstorms include single cell or ordinary cell, multicell clusters, and multicell lines.
According to this very informative site:
Supercell thunderstorms are among the most dangerous thunderstorms that occur on the planet, and almost always produce severe weather of one kind or another. Ordinary thunderstorms can pass by with no more than a few rumbles of thunder and a heavy, but brief, deluge of rain. The supercell, though, is a factory for many of the most damaging weather types, ranging from large hail to vast wind gusts to numerous lightning strikes to some of the most powerful tornados recorded.So it is some phenomenon that no one likes to ever encounter it or to see it happening. Somehow the above picture posted recently in Twitter with no indication of where or when it was taken. Was it overland or over thwe ocean? Whatever, may there will never be a supercell ever occur in your/mine/our life paths.