5301991f_and_The_Art_of_Breaking_Things__detailed_explanation
[signal] Hand clamped over mouth [part of body] mouth / hands [possible meaning(s)] suppression, holding back, shock [detailed explanation] 
Often an unconscious gesture of self-regulation – stopping speech for reasons of shock, embarrassment, or for more tactical reasons. The gesture is reminiscent of the ‘speak no evil’ wise monkey. The action can be observed very clearly in young children when they witness something ‘unspeakably’ naughty or shocking. Extreme versions of the same effect would involve both hands.
“I pretty much try to stay in a constant state of confusion just because of the expression it leaves on my face.” – Johnny Depp | Zen and The Art of Breaking Things
detailed explanation