In Javascript, what does this underscore mean?

It means private fields or private methods. Methods that are only for internal use.

They should not be invoked outside of the class.

Private fields contain data for internal use.

They should not be read or written into (directly) from outside of the class.

Note: It is very important to note that just adding an underscore to a variable does not make it private, it is only a naming convention.

As far as I'm aware, it's generally used to indicate a private variable (but doesn't actually provide any privacy, just a convention).

It's discussed briefly here, though they're advised against: http://javascript.crockford.com/code.html