Tibetan Buddhism identifies six 'bardos', or states of consciousness, associated with the phases of life and death. Early in Buddhist history, the word 'bardo' generally only referred to the phases after death and between lifetimes, and in common usage it is still often used this way. Early Buddhists differed on whether rebirth occurred immediately after death, or whether there was an interval and transitional period. When Mahayana Buddhism developed later on, including both Tibetan Buddhism and Zen, most lineages accepted the idea of an interval, and the teachings on bardos developed. Within Tibetan Buddhism and some other Mahayana traditions, three bardos experienced during physical incarnation were added to the original three after-death or between-life states.
The six bardos are:
First bardo (Shinay): Our lifetime in physical form, lasting until our consciousness or mindstream leaves our body at death (which in Tibetan teachings does not occur until sometime after we are physically considered deceased.)
Second bardo (Milam): Our dream experiences and dream states, occurring within a lifetime of the first bardo. One branch of Tibetan practice commonly referred to as 'dream yoga' focuses on how to transform the dream state into Buddhist practice.
Third bardo (Samten): True meditation, also occurring within a lifetime in the first bardo. This particularly refers to states reached by experienced meditators who are practicing correctly, although some may have a spontaneous experience of it outside meditation.
Fourth bardo (Chikkhai): The death transit, in which we experience the 'clear light', followed by unconsciousness. Tibetan descriptions of this bardo are often compared with descriptions of after-death or near-death experiences in other spiritual traditions. In Tibetan teachings, this state begins slightly before what we think of as physical death, and continues for approximately three days until complete dissolution or transmutation to the next bardo has occurred. An experienced Buddhist practitioner, or one whom has absorbed the teachings of the Tibetan Book of the Dead, may attain liberation in this phase.
Fifth bardo (Chonyid): A series of dream or trance-like visions and auditory sensations that each being experiences differently based on their remaining karmas, particularly their intense aversions or desires. An experienced Buddhist practitioner, one able to recognize the clear light of the fourth bardo (even if liberation was not attained there), will be able to maintain an inner equanimity during this phase, and even experience transcendent realms of being, while others will be trapped in full delusion, as if immersed in a movie or dream.
Sixth bardo (Sidpai): The process of becoming or being reborn. Based on remaining karmas, a being will experience more visions that will eventually propel them into a new lifetime - back to the first bardo. Liberated beings that are returning based on a bodhisattva vow experience this bardo, and the fifth, in full awareness, until the commencement of the first bardo, when they lose their full past-life recall, just like other beings (although some subtle memories may remain, or be recalled at a later time.)
The Tibetan Book of the Dead, or Liberation Through Hearing During the Intermediate State, is a treatise from the Nyingma school of Tibetan Buddhism designed to guide an individual through the death process and the interval prior to rebirth. This series of teachings is typically read aloud over a dying, or recently deceased person, and/or studied by practitioners during life. Sogyal Rinpoche's bestselling The Tibetan Book of Living and Dying introduces these teachings in an accessible way to a Western audience.
However, teachings on the bardos are not only considered useful for navigating the death planes. All of the six bardos can be understood as transitional states, and understanding them and their inter-relationship is a key practice within many Tibetan Buddhist lineages. In a sense our consciousness is in transition every moment, and learning to recognize these shifts, and more importantly the ground in which they occur, is essential practice.