24/30 geting confused whti "this"

I´m getting confused whit “this” maybe somebody could give me a hint. i understand that insider a constructor “this” is used to assign the “input” value to the value insider the constructor.

But what happens here: this.getBalance = function() what technical “this” means here? and even more general what does “this” do?

  function Person(first,last,age) {
   this.firstname = first;
   this.lastname = last;
   this.age = age;
   var bankBalance = 7500;
  
   this.getBalance = function() {
      // your code should return the bankBalance
      return (bankBalance);
   };
}

var john = new Person('John','Smith',30);
console.log(john.bankBalance);

// create a new variable myBalance that calls getBalance()
var myBalance = john.getBalance();

console.log(myBalance);

the code will also work when i remove “this” from the constructor.

function Person(first,last,age) {
  firstname = first;
  lastname = last;
  age = age;
   var bankBalance = 7500;
  
   this.getBalance = function() {
      // your code should return the bankBalance
      return (bankBalance);
   };
}

var john = new Person('John','Smith',30);
console.log(john.bankBalance);

// create a new variable myBalance that calls getBalance()
var myBalance = john.getBalance();

console.log(myBalance);