What is a prototype in JavaScript?

A prototype in JavaScript is a mechanism that allows objects to inherit features from other objects. It's a property (in the form of an object) that is shared among all instances of an object type.

Here's a step-by-step explanation:

1. In JavaScript, every function and object has a "prototype" property. This property is a link (or reference) to another object. When a reference to a property or method is made, JavaScript will search in the object's own properties first. If it doesn't find it there, it will then look in the object's prototype.

2. The prototype property is used primarily for inheritance; you add methods and properties on a function’s prototype property to make those methods and properties available to instances of that function.

3. When you create a new object using a constructor function, the newly created object inherits the properties and methods from the constructor's prototype.

4. The Object.prototype is on the top of the prototype inheritance chain. All objects will inherit properties and methods from Object.prototype.

For example, if you have a function called Car and you want all instances of Car to have a method called drive, you would add that method to Car.prototype.

function Car(make, model) {
  this.make = make;
  this.model = model;
}

Car.prototype.drive = function() {
  console.log(this.make + " " + this.model + " is driving!");
}

var myCar = new Car("Toyota", "Corolla");
myCar.drive(); // logs "Toyota Corolla is driving!"

In this example, myCar is an instance of Car and it can use the drive method because it's in Car's prototype.