If nothing else it has an amusing name, it’s pronounced “dunder proto” due to the double underscore notation it borrows from python.
The __proto__ property is used to access the prototype chain for an object instance. Don’t know what a prototype chain is? Go take a look here.
So where does it come from? It gets created on any new instance during construction. If construction is done using a constructor function i.e by using the new keyword
var a = new Person()
it will point to the prototype of the constructor function, in this instance Person.prototype. If done using object literal notation var b = {} it will point to Object.prototype. Note that as everything in JavaScript is an object, all objects can follow their prototype chain back to Object.prototype
You can also gain access to the prototype using Object.getPrototypeOf(...). MDN provides more on this. You should give this a read if you haven’t already to understand the history of this property.
The following code demonstrates the setup of two very simple object literal and constructor function prototype chains.
//Object literal
var o = {};
o.__proto__ === Object.prototype; //true
o.prototype === undefined; //true
//Constructor function
function Shape(){}
var circle = new Shape();
circle.__proto__ === Shape.prototype; //true
circle.__proto__.__proto__ == Object.prototype; //true
Shape.__proto__ === Function.prototype; //true
More reading.
I’ve already linked them above but as always the MDN docs are a good place to start. There is a great stackoverflow post that has some really valuable insight amongst the many answers and comments.