Taking refuge is the first step to become a Buddhist, entrance to all buddhist practices. It is a commitment to accept the three jewels, Buddha as the guide, Dharma as the path and Sangha as companions, on the road to enlightenment.
In Vajrayana, Buddha, Dharma and Sangha are the Outer Refuge. Three Roots: Lama, Yidam and Khandro are the Inner Refuge. The Three Bodies, or Trikaya, are the Secret Refuge.