Could be for a number of things, usually either a website or to log on to certain computer networks (like your office PC). A couple websites that use that technology (although maybe not that same brand) are PayPal or E*Trade.

Anyway, it's one level of authentication. Your normal username & password are things you KNOW, technology like biometrics use something that you ARE (that is, the physical size/shape of your face, hand, etc.), and a physical device like that password token is something that you HAVE.

A hacker or even a mischievous office-mate that discovers your password can't get in while you still have physical possession of your password fob.