Originally when the Tibetans began using the face of Mahakala for protection, they were a nomadic people, living in tents and moving from here to there all the time. He is still often referred to as "Lord of the Tent", as roughly 40% of the Tibetan population is still nomadic or semi-nomadic, though now the mask of his face frequently hangs on more permanent walls as well. Because he is considered a protector of religious law in particular, Mahakala masks are frequently found in temples and monestaries in Tibet, as well as in private homes.
The blaze of fire above Mahakala's eyes and coming from his mouth represents his powerful energy for consuming neurotic states of mind. His third eye denotes him as a protector; three orbs of vision express his ability to see the past, present and future. Staring at the world with widened eyes signifies that he is incensed at the current state of affairs, and none shall pass him without being affected.
Mahakala has a crown of five skulls, which represent the transmutation of five negative afflictions of human nature into virtues. As he confronts negative forces and crushes them with his anger, his crown of skulls neutralizes what remains and works to transform it into something positive. Thus ignorance becomes wisdom, pride is humbled, attachment becomes the wisdom of discernment, jealously shifts to feeling satisfied with one's own accomplishments, and anger is released and soothed.
Consider Mahakala your spiritual bodyguard. He appears to be tough and larger-than-life scary, but he is somebody you want to have on your side as you face the trials and stresses of life. That face is angry only in the service of a greater good: your happiness, wisdom, and karmic health!