Arnold van Gennep described its three phases: separation, liminality, incorporation. One steps out of the ordinary world (separation), passes through a time of threshold and dissolution (liminality), and is received back as someone new (incorporation).
Traditional initiations handle major transitions: puberty, marriage, ordination, death. The modern world has largely lost them, though fragments persist — graduation, confirmation, bootcamp. What is not handled ritually is often handled by the psyche without containment, at cost.
The contemplative path is, among other things, a long series of small initiations.