JS面向对象问题的请教！

Question

// 定义Person构造器function Person(firstName) {  this.firstName = firstName;}Person.prototype.walk = function(){  alert("I am walking!");};Person.prototype.sayHello = function(){  alert("Hello, I'm " + this.firstName);};function Student(firstName, subject) {    Person.call(this, firstName);//  请教 call函数的作用 以及（this,firstname）两个参数的含义   this.subject = subject;};Student.prototype = Object.create(Person.prototype); // 这段不懂 请教