The Tibetan word bardo (བར་དོ་ Wylie: bar do) means literally "intermediate state"—also translated as "transitional state" or "in-between state" or "liminal state". In Sanskrit the concept has the name antarabhāva. It is a concept which arose soon after the Buddha's passing, with a number of earlier Buddhist groups accepting the existence of such an intermediate state, while other schools rejected it.
Used loosely, the term "bardo" refers to the state of existence intermediate between two lives on earth. According to Tibetan tradition, after death and before one's next birth, when one's consciousness is not connected with a physical body, one experiences a variety of phenomena. These usually follow a particular sequence of degeneration from, just after death, the clearest experiences of reality of which one is spiritually capable, and then proceeding to terrifying hallucinations that arise from the impulses of one's previous unskillful actions. For the prepared and appropriately trained individuals the bardo offers a state of great opportunity for liberation, since transcendental insight may arise with the direct experience of reality, while for others it can become a place of danger as the karmically created hallucinations can impel one into a less than desirable rebirth.
The term bardo can also be used metaphorically to describe times when our usual way of life becomes suspended, as, for example, during a period of illness or during a meditation retreat. Such times can prove fruitful for spiritual progress because external constraints diminish. However, they can also present challenges because our less skillful impulses may come to the foreground, just as in the sidpa bardo.(Wikipedia)