IDP is “International Driving Permit”, which is a booklet that certifies that the person has a driving license in <country> for <class of vehicles>. It includes the photo of the person and the date of birth. It includes translation in various languages so that when you’re in another country and rent a vehicle (or drive one) and the police (or enforcers) need to check if you have a valid license, you can show this (as well as your original license, if asked for). Anecdotally, I’ve heard that the process to get it through AAA seems quicker and easier than from the DMV.