The layer where every human activity became a venture-backed destination, every destination became a feed, every feed became ad inventory, and every ad market became a machine for producing more things to interrupt you with.
Underneath that layer is another internet: older, slower, less polished, harder to monetize, and much harder to kill.
It is not utopia. It is full of spam, abandoned servers, broken clients, hostile nodes, strange old commands, half-maintained software, and people arguing in plain text about things no normal person should care about.
You may know it by command prompt or terminal. The shell is what’s actually executing the commands you type in there. Bash is the most commonly used one for most Linux systems, zsh is the default for Mac in recent years, but there’s others like fish, etc.
The shell you use also determines the syntax so if you use one, scripts meant for another might not work.