It is usually typical American hyperbole (extreme exaggeration). True 'freaking out' would be a visibly manic reaction (screaming, yelling, destroying things) in reaction to some statement/incident. But - in the same way that when someone says/writes ROFL (Rolling on the floor laughing) , they probably only actually twitched a faint smile, 'freaking out' usually would really refer to someone expressing disapproval at the aforementioned statement/incident.

1. How you react when something really bad or good happens. A manic, crazed feeling usually accompanied by some physical activity such as jumping, skipping, fainting, fluttering, screaming. Example: After working so hard for weeks, when I found out I passed the final test, I just freaked out! 2. to shock or disorient someone 2. to panic, to lose control Example: "Woah, don't freak out, it's not the end of the world."