Whats the meaning of the expresion "What the Hell?"?

Answer 1

It actually can be used in a variety of ways, depending on the emotion expressed with the voice. Note the following listing at dictionary.com

24. what the hell, Informal. (used to express lack of concern or worry, indifference, abandonment, surrender, etc.): As long as you're borrowing $100, what the hell, borrow $200.
http://dictionary.reference.com/browse/hell

Other uses - to express anger and/or shock/disbelief, sometimes disgust.
"What the hell do you think you're doing?!"

In other words, it can also communicate that the speaker finds something beyond belief and/or unacceptable.

Answer 2

Utter bewilderment.