Hi I'm trying to make a register route in my page using passport. What I can't understand is

What passport.authenticate("local")(req,res,function(){}) is

I understand passport.authenticate("local") is by using local strategy, we judge id/password is right or not. But I have no idea how (req,res, function(){}) after that part works. How can we write like that?

+) "User" in the code is just a model name.

app.post(“/register”, function(req,res){ 
User.register({username:req.body.username}, 
req.body.password, function(err,user){

  if (err) {
    console.log(err);
    res.redirect(“/register”);
    }else{
    passport.authenticate(“local”)(req,res,function(){
      res.redirect(“/secrets”);
});