What is the function of router(app) do in the following code?

Here are few lines of code.I did not understand their function.I have commented in the code the lines that I have not understood.

 var express = require('express');
 var app = express();
 var router = require('./api/router'); //not understood
 router(app); //not understood

It will be helpful I anyone can explain their function.

via Aditya Jain