Can you please explain this to me like I'm a brand new butterbar (which I am)? In the words of the guy from Margin Call, speak to me as you would a young child - or a golden retriever. What work can qualify as an additional drill, and how much does there need to be to do so?
An additional drill is a drill (not active duty orders) that is beyond your entitlement (which is 48 IDTs/year). There are several flavors, depending on what you do. ATPs are for non-fliers, AFTPs are for fliers and must be used for either a flight or sim event (and some additional wiggle room depending on the FY instruction), and RMPs are admin drills that can be done from home or at your Reserve work. RMPs are limited to 12 per year (?...it's been a while, is that still correct?). AFTPs are limited to 72. I can't remember what ATPs are limited to (36?).
IDTT, as @MIDNJAC mentioned, is a set of orders, but not active duty orders, where you can use IDTs or AFTP/ATP drills away from your home unit. Basically it's funding that pays for the travel/lodging/per diem while you're executing drills. It's a sweet deal, but not always available due to funding limitations.