I didn't understand what is a method?

Answer 1

Concisely, just like properties which are variables of an object, methods are functions of an object.

Answer 2

method is 'what action would you associate with that particular object'. for example: while creating an object called clientDetail, you would want to associate 'actions' such as updating the details. so an 'updateDetail' would help you here. this allows you to 'attach the functionality' of updating the details with that object.