someone please explain me how this error handler working here in app.js

var express = require('express');
var bodyParser = require('body-parser');
var mongoose = require('mongoose');
var session = require('express-session');
var app = express();

//mongodb connection
mongoose.connect("mongodb://localhost:27017/bookworm");
var db = mongoose.connection;
//mongo error
db.on('error',console.error.bind(console,'connection error : '));

//use session for tracking logins

app.use(session({
    secret: 'treehouse loves you',
    resave : true,
    saveUninitialized : false
}));

//make userId access in all templates
app.use(function(req,res,next){
  res.locals.currentUser = req.session.userId;
  next();
});

// parse incoming requests
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));

// serve static files from /public
app.use(express.static(__dirname + '/public'));

// view engine setup
app.set('view engine', 'pug');
app.set('views', __dirname + '/views');

// include routes
var routes = require('./routes/index');
app.use('/', routes);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('File Not Found');
  err.status = 404;
  next(err);
});

// error handler
// define as the last app.use callback
app.use(function(err, req, res, next) {
  res.status(err.status || 500);
  res.render('error', {
    message: err.message,
  });
});

// listen on port 3000
app.listen(3000, function () {
  console.log('Express app listening on port 3000');
});

router.post('/login',function(req,res,next){
    if(req.body.email && req.body.password){
        User.authenticate(req.body.email,req.body.password,function(error,user){
            if(error || !user){
                var err = new Error('Wrong email or password.');
                err.status = 401;
                return next(err);
            }else{
                req.session.userId = user._id;
                return res.redirect('/profile');
            }
        });
    }else{
        var err = new Error('Email and password required.');
        err.status = 401;
        return next(err);
    }
});

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('File Not Found');
  err.status = 404;
  next(err);
});

// error handler
// define as the last app.use callback
app.use(function(err, req, res, next) {
  res.status(err.status || 500);
  res.render('error', {
    message: err.message,
  });
});