The add-on sends you an iMessage and/or a sound from your Mac(book), if your render is ready.

You can find the add-on in the viewport by press N to open the sidebar and then in tools -> iMessage after render.

In the text field you can use the following commands:

In the newest version, now the default text is "Render ready: __filename__".

Also the phone number is now globally stored so you don't have to enter it for every scene.