Overriding Methods

I don't understand how to do this... can someone help and explain so I can better understand? Task: I've provided a base class Person in the editor below. Once an instance of Person is created, you can call getFullName() and get a person's full name.

Your job is to create a class named Doctor that overrides the getFullName() method. Once you have a class definition, create an instance and assign it to a constant named someDoctor.

For example, given the first name "Sam", and last name "Smith", calling getFullName() on an instance of Person would return "Sam Smith", but calling the same method on an instance of Doctor would return "Dr. Smith".

class Person {
    let firstName: String
    let lastName: String

    init(firstName: String, lastName: String) {
        self.firstName = firstName
        self.lastName = lastName
    }

    func getFullName() -> String {
        return "\(firstName) \(lastName)"
    }
}
let fullName = Person(firstName: "Sergio", lastName: "Romero")
// Enter your code below
class Doctor: Person {

    override func getFullName() -> String {
    return "Dr. \(lastName)"
    }
}

let someDoctor = Doctor.getFullName