Aromatase Inhibitor. They reduce estrogen in the body by limiting the conversion of testosterone to estrogen (which is how estrogen occurs in the body naturally). When you take synthetic testosterone higher than normal physiological levels, your body converts more of it to estrogen. You want to limit the conversion, so that your estrogen stays at normal levels while your T is high. Clomid doesn't do this. Products like Arimidex and Aromasin do. I would recommend Arimidex.
I would also recommend you continue reading before you do anything. AI's come up in pretty much ALL discussions about beginner cycles. The fact that you didn't know what they were is really disconcerting. You need to learn more.