Inside the Meta Script Data Source edit page click on the small "?" you will see an explanation there. Here is a excerpt of it:
JavaScript / ECMAScript
This data source requires some knowledge of JavaScript, also known as ECMAScript. (The full specification for ECMAScript is available here.) JavaScript is arguably the most popular scripting language in use today, it being the only language available for use in every popular web browser. The result is that, by far the common usage of JavaScript is in web pages. The context within the Meta data source is of course different, there being no "window" or "document" objects, among other more subtle differences. Contextual differences aside, a full and complete implementation of ECMAScript is available within this data source. Specifically, the Rhino implementation is used https://developer.mozilla.org/en-US/docs/Rhino.