It's the ground. It prevents that thing from frying during a surge, or from popping your breakers. You should see a titanium screw somewhere in the housing for all of that. Wrap it around that screw, and twist it tight to secure the wires. This is usually how the ground works, at least in my inline fan.
Google Search could actually give you an excellent guide on how to wire something up, and what all the colors mean. Most people that know good information like this usually pass over threads like this because it's a question that has probably already been answered a few times on here. I'm not saying they should pass over, or that it's your fault this information has already been posted here, but I am letting you know that is how it works.
Also, how big is your space? That fan is a duct booster. Not an inline fan. If you have a small little cabinet it should be fine, but if you have a tent or a room, as well as a carbon filter, that is not sufficient. A inline fan is usually only 40-60 bucks on top of a duct booster. Well worth it. My best advise is to never travel to the hydro store with less than 350 saved up, or you will be constantly upgrading your purchases. Well, unless you are running out for a clone tray or bug killer, as I often do.
