See the MDN docs here for a description and example code:
A getter is a method that gets the value of a specific property. A setter is a method that sets the value of a specific property. You can define getters and setters on any predefined core object or user-defined object that supports the addition of new properties.
As noted in the docs (and by @ cwallenpoole), __define[GS]etter__() functions are now deprecated. There’s a lot more detail in this article. I believe the defineProperty() function is now the preferred syntax.