The Full Form of AGI is Advanced Ground Instructor.

An Advanced Ground Instructor (AGI) can do everything a BGI can, plus provide the ground instruction necessary for any certificate except the instrument rating. You can also endorse students to take the FAA knowledge tests for those ratings, where applicable. Earning the AGI provides more privileges and requires minimal additional studying. Typically Gleim recommends getting the AGI if you are debating between basic and advanced. You do not need to get a BGI prior to obtaining an AGI. The questions on the AGI exam are similar to those in the question pool for BGI, but there are a few additional topics that pertain to pilots training for advanced certificates. The exam also has 20 more questions, but the same 2.5 hour time limit.