Essentially an AAR is a reflection. After a mission or any type of event involving many people, the leader asks all key personnel to meet up at some point soon after the event. The whole point was to get the thoughts of those personnel so the group was able to learn from the each others mistakes and take note of each others success in that event. The common format was to give 3 Go's (things to keep the same), 3 No-Go's (things to change/get rid of) and 3 improvements (I sure hope you know what an improvement is).
As an enlisted soldier I hated these with a passion because they always occurred right before you were to be released and all I wanted to do was sleep/eat/have a beer/get home. But now that I'm an officer and have more "responsibility", every AAR is an oppurtunity for me to A) see where I jacked something up and B) see where something made a bigger impact than I expected.
Now to the point, drum roll please........GO LEARN SOMETHING! Did you just learn the CTRL+P prints your current page? Fantastic, you just saved yourself the excruciating pain of moving your cursor to File>Print. I just found out that there is a farmers market 2 miles from my apartment that offers fresh, local and sometimes organic produce/meat cheaper than my local grocery store. That little tidbit of information has allowed me to lower my grocery bill and eat healthier.
Does it always have to be something related to your career or hobbies? Not at all. Will you get more use and excitement out of something related to your career or hobbies? You bet. Just don't be to upset if you find yourself learning a bunch of unrelated tidbits, at least you'll improve your "Trivia Night at Hooter's" performances.