-A digital trigger (shown in the pic) only has two degrees, pressed and not pressed.
-Whereas an analog trigger has many degrees of pressure as you push it down (or 'pull' it down, if you wanna be a (0-0) lol)
As Cerberuz said, its great for things like racing games when you dont want to floor it into a turn or something. A little press on the trigger makes a huge difference. Better control. Just hope the final controller has em.