What does such a code mean?

Question

Node.js

I can't figure out what it means res.redirect? and even more incomprehensible what he does in conjunction with return?

exports.getLogin = (req, res) => {
if (req.user) {
    return res.redirect('/'); // 
  }
  res.render('account/login', {
    title: 'Login'
  });

Then this construction is used in the app.js file like this:
app.get('/login', userController.getLogin);

Answer 1

Have you tried reading the documentation?
expressjs.com/en/api.html#res.redirect
And in combination with return it does the same thing as

res.redirect('/');
return;

Together with the condition and res.render at the end, this gives:
If the user is logged in, then we redirect him to the page /, otherwise we give him the login page.

Answer 2

return is needed here to exit the getLogin function before rendering the login page, provided that the user is logged in.