Can someone explain the numbers associated with xbox controller triggers?

This is to help you setup an xbox controller(win/mac are different as described in the link) within the InputManager.

While unity is open, click on Edit -> Project Settings -> Input

Expand the drop down for axes.

You can Define all the controls within this InputManager. It has default Input settings for the mouse and some keyboard input. You will need to expand the Element Size and define the input according to the layout/maping from that page.

Edit:

What are the numbers on the graphic? It is associated with the Joystick button number that needs to be defined into the input manager. There isn't a positive or negative number for the buttons. You can see in the default input manager entries that they use "joystick button 0" this is the A button on the xbox 360 controls and also indicated on that graphic with 0 over the A button.