FACE PALM is essentially a physical gesture which is the equivlent to slapping your forehead with the palm of your hand, concealing part of your face. The gesture is found in many cultures as a display of exasperation.
On the Internet, the term is used as an expression of embarrassment, frustration, disbelief, disgust, shame or general woe. It often expresses mockery or disbelief of perceived idiocy.