Why do we need to declare the same value and type twice in classes with Interface in Typescript

Question

I am just getting started learning TS these days. I have a question about class and Interface.

According to below code,

// Classes with Interfaces

interface PersonInterface {
  id: number;
  name: string;
  register(): string;
}

class Person1 implements PersonInterface {
  id: number;
  name: string;

  constructor(id: number, name: string) {
    // console.log(123); this could be console whenever the object run
    this.id = id;
    this.name = name;
  }

  register() {
    return `${this.name} is now registered`;
  }
}

class Employee extends Person1 {
  position: string;

  constructor(id: number, name: string, position: string) {
    super(id, name);
    this.position = position;
  }
}

const emp = new Employee(3, "Jiwan", "Developer");

console.log(emp.name);
console.log(emp.register());

I am not sure Why do we need to declare the same value and type twice in classes with Interface. If I have to declare here and there without any reason, I understand how does TS works. However, I am wondering if there is a specific reason here.

In advance, I am so sorry If I ask a stupid question. I am a pretty beginner. Thank you for your help!