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I could suggest a quick search on Enterprise Activation, since this question has been asked a few times before (OK, a lot of times), but here's the quick explanation.
Enterprise Activation is used to set up BlackBerrys for use on corporate BlackBerry Enterprise Servers (v4.0 or higher) and provision them over the air. Unless you are on BES 4.0, you shoudl hide this icon.
Good luck and welcome to BB Forums!