A Sell-Zone or supply zone is a very specific zone where two things are true:
1. A large amount of Sell orders originated there.
This just means that, within this Sell-Zone, market makers clicked sell, with the volume of thousands and thousands of lots.
2. A large amount of Sell volume is sitting there.
This just means that, within this Sell-Zone, there exists demand for Sell order , if price were to return to this Sell-Zone in the future. This Sell zone or supply can be in the form of existing pre-set limit-orders, as well as new supply of Sell volume that may potentially enter when price returns to this Sell-Zone.
Again, The markets are algorithmic, and the buying and selling is nothing more than the initiation of buy programs and sell programs. The majority of the financial markets are almost entirely automated and algorithmic, with very little human interference. This makes them more predictable, but more dangerous