Fermented tea is a class of tea that has undergone microbial fermentation, from several months to many years

The exposure of the tea leaves to humidity and oxygen during the process also causes endo-oxidation (derived from the tea-leaf enzymes themselves) and exo-oxidation (which is microbially catalysed)

First, young leaves are picked from a plantation. Tea leaves are selected to go through the fermentation process, which involves steaming for approximately 5 minutes, removing the remaining water, selecting tea leaves again, packing them into clay pots, and pressing the leaves by heavy weights.

 The fermentation process needs to be checked at intervals. The tea leaves are well fermented by natural forming microbes. The pulp softens after 2 weeks and the fermentation process is completed in 3–4 months. The completion of fermentation is evident when the pulp changes from green to golden green, the leaves soften, and acidity is reduced. 

The unit flavor develops because of the phenolic compounds in laphet. Furthermore, there is assumption to be found in the steaming of fermented tea leaf processing that steaming makes the increased production of phenolic and some compounds in tea leaf and become amenable for particular fermenting microbes to grow, while other potentially unwanted microbes cannot grow, even if the fermentation is conducted under non-sterile conditions.

https://www.sciencedirect.com/science/article/pii/S2352618115000645
https://en.wikipedia.org/wiki/Fermented_tea