Chrome Console Object Properties Shown with Angle and Square Brackets
Most of what you're asking is answered here. Just like that answer I'll go ahead and quote the spec:
They are defined by this specification purely for expository purposes. An implementation of ECMAScript must behave as if it produced and operated upon internal properties in the manner described here. The names of internal properties are enclosed in double square brackets [[ ]]. When an algorithm uses an internal property of an object and the object does not implement the indicated internal property, a TypeError exception is thrown.