He probably meant The moment after doing this, the addMethod becomes available for all basic objects object types like String, Number etc. This is because the String object is a function (but objects created by String are not).
E.g., given
var s = '';
You can do
String.addMethod(...);
but not
s.addMethod(...);
A brief explanation of the JavaScript type system comes here:
JavaScript does not have the normal concept of classes. Instead, you can achieve somewhat the same by turning any function into a constructor by puttin the new keyword in front of it when it is invoked.
E.g: given
function MyFunction(x) { this.myX = x; }
if you invoke it like
var myObj = new MyFunction(10);
it will create an object called myObj. This object will have a single member variable called myX. The function MyFunction is considered the constructor of the object (and is stored in the "constructor" property.
(Bonus question: What will happen if you invoke the function above without the new keyword, i.e. var x = MyFunction(10)
. The answer will probably surprise any sensible person.)
Now you have seen how any arbitrary function can be turned into a constructor. The built-in objects are exactly the same, string objects are created by the function String, numbers are created by the function Number, etc.
Just as those built-in objects are created by functions, each of those functions are also created by the "Function" function (yikes!).
Now on to prototypes.
in the example above, if you somewhere write
MyFunction.prototype.someNewMethod = function() {}
all objects created by the MyFunction constructor/function will appear to have an extra member function called someNewMethod. You can do many other funky things with prototypes, like replacing the prototype, or replacing the prototype's prototype, but I'm not an expert at that.