express()

When you require a module require('some_module'); it will search for the specified file, and make the exports object from that file available to your program. For example, just say in the file square.js, there was this code inside

exports.area = function(width) { return width * width; };
exports.perimeter = function(width) { return 4 * width; };

So if I were to write

const mySquare = require('./square');

it would let me directly use those functions i.e. writing

console.log(mySquare.area(4));

would print the value 16.

Now that we understand how modules work, why do I have to use 2 declarations for my Express app, like so?

const express = require('express');
const app = express();

Well, when you require express, it returns the function "createApplication", you can see that if you go to line 28 here. You now have the function in your program, but in order to create the app, the function has to actually run. Therefore it is standard practice to do this in two lines, the require line loads the function, and the second line will run the function. However, it does not have to be written this way, for instance, combining both lines into one like this will also work

const app = require('express')();

However, I recommend sticking to the accepted convention of creating the app in 2 lines, even if it isn't strictly necessary. Hope that helps