Understanding passport serialize deserialize

How would you explain the workflow of Passport’s serialize and deserialize methods to a layman.

1. Where does user.id go after passport.serializeUser has been called?

2. We are calling passport.deserializeUser right after it where does it fit in the workflow?

   // used to serialize the user for the session
   passport.serializeUser(function(user, done) {
       done(null, user.id); 
      // where is this user.id going? Are we supposed to access this anywhere?
   });
   
   // used to deserialize the user
   passport.deserializeUser(function(id, done) {
       User.findById(id, function(err, user) {
           done(err, user);
       });
   });
   

I’m still trying to wrap my head around it. I have a complete working app and am not running into errors of any kind.

I just wanted to understand what exactly is happening here?

Any help is appreciated.