tibetan skull tattoo

Tibetan Buddhism is a form of Tantric Buddhism. Tantra started in India and then spread to Tibet. Tantric practices try to stop our attachment to our bodies. Many tantric practitioners would meditate in grave yards. In Buddhism all things are impermanent and will change. Your body will die and decay and it is your attachment to your body that will tie you to this world of suffering. Therefore if you practice you will free yourself and enter into Nirvana. Sorry answer is long but it is complex area.