What is the second argument in an Express get request? code example

Example 1: express get url parameters

app.get('/path/:name', function(req, res) {
  res.send("tagId is set to " + req.params.name);
});

Example 2: express router file

var express = require('express');
var router = express.Router();

// middleware that is specific to this router
router.use(function timeLog (req, res, next) {
  console.log('Time: ', Date.now());
  next();
});
// define the home page route
router.get('/', function (req, res) {
  res.send('Birds home page');
});
// define the about route
router.get('/about', function (req, res) {
  res.send('About birds');
});

module.exports = router;