AI is an aromatose inhibitor. In a nutshell it restricts how much testosterone is converted to estrogens by the body.
And not to scare the shit out of you or anything but I have never been able to achieve the quality of erection that I had prior to using gear and that was four almost five years ago now (last cycle). I followed PCT and everything down to the letter but your body gets used to having a large amount of testosterone in it and for some individuals, myself included, seemingly never recover from this.
I have no problem getting an erection but its not as hard as it used to be (I used to feel like I could use it as a hammer now its just... meh), and it is difficult to maintain without constant stimulation. I have tried everything in the book to try and get my performance back to where it needs to be. I know this isn't the case for everyone out there and there are probably a lot of scoffers at this point thinking I didn't do something right but when you fuck with your bodies natural chemical balance, there is the chance, as in my case, that you never fully recover in some aspect.
That said, my nuts are as big as ever and my test and free test levels are in the high normal range... it took them a few months to get back to there glorious size but part of me wishes that I had never touched the juice simply for the fact that its permanently damaged my sexual performance. Best of luck to you!