Basic Node and Express: Start a Working Express Server - running in background?

In this lesson it says that

" For testing reasons, we need the app to be running in the background so we added this method in the server.js file for you."

I’m curious as to what this means. I can see that there is some code that seems to set up a background app:

bGround.setupBackgroundApp(app, myApp, __dirname).listen(port, () => {
  bGround.log(`Node is listening on port ${port}...`);
});

Does running in the background mean, perhaps, that it is running on a separate thread? I know that JavaScript is single threaded right? So I guess server.js is running on one thread in the Node.js runtime environment, and my guess is our Express app will be running on another?

Sorry if my terminology might be off, this (server side development - full stack development as a whole really, but I definitely have a much less clear understanding of how back end web servers work compared to the client side) is all very new to me. Please correct me if any of my verbiage is off, thanks in advance!