The underscore is simply a valid character in an identifier, so the method's name is _render.
It's a common pattern in languages without access modifiers to use underscores to denote private methods. In a language such as C#, which does have access modifiers, I could define a method as:
private void Foo() {}
The method Foo can then only be called from within the class which defined it.
In JavaScript you can't do this, so it's a typical design pattern to prefix the method with _ to show that it should be treated as private.
this._foo();
You can still call this method, it's just not considered good practice to do it outside of the class definition.
