Routing refers to determining how an application responds to a client request to a particular endpoint, which is a URI (or path) and a specific HTTP request method (GET, POST, and so on).

Each route can have one or more handler functions, which are executed when the route is matched.

The following example illustrates defining simple route. Respond with Hello World! on the homepage:

app.get('/', (req, res) => {
  res.send('Hello World!')
})

Source:

Express Basic Routing. expressjs.com/en/starter/basic-routing.html.