Just what the question above says, what is an 'Association Bonus'? And why can't I click on it? I received loads of extra reputation just out of nowhere because of it... (p.s. I'm not complaining, please don't take it away, I'm just curious)
If one of your stackexchange accounts has 200+ reputation points, then you get +100 rep points in any new site that you join. This allows you to get past restriction on some basic features (such as commenting, flagging, upvoting). Stackexchange believes that if you have gathered 200 reputation on any site then you are acquainted with the basics of stackexchange and therefore a (somewhat) trusted user.