It's the mute key for phone calls and for music it will pause what's playing. Most BBs have that key as Auto Standby, but not the Storm. (Unless 328 has that feature; I haven't updated yet.)
Well when your on a call and you press it, it should mute the call,
Stoping and playing music,
Or if you have your phone locked (with the blackberry factory lock) and you hold it down for about 5 seconds it will change your profile to vibrate and it you repeat it will change your profile back to whatever you had it before.