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Before OOP languages, the programmer would control how the program is executed and that would be from the app’s main routine. In modern programming, that control is delegated to the external non-main loops, and the main routine becomes an event-loop that waits for events to occur and then executes it with the relevant event handler.
This new model of programming (it has been around since the 70s though) is called event-driven programming.
Concurrency vs ParallelismNode.js has long excelled at concurrency. With the recent release of Node 13.0, Node now has a stable answer to parallelism as well.
Concurrency can be thought of as switching between async processes, which all take turns executing, and, while idle, return control back to the event loop. On the other hand, parallelism is the ability for a process to separate and run simultaneously on multiple threads. There are other solutions in JavaScript that have tried to address this problem. For an in-depth comparison, I found this article useful.