This is a question that lives entirely inside a belief system, and any honest answer has to begin there. The idea that a soul passes from one life to the next, keeping some things and shedding others, belongs to spiritual and religious traditions, not to anything science has measured or confirmed. There is no evidence that a soul exists in this sense, that incarnations occur, or that anything is carried across them. So the most accurate framing is not what the soul does retain, but what various traditions and practitioners say it retains.
Within past life regression and the spiritual frameworks around it, the answer tends to follow a recurring shape. What is said to be kept is usually described as essence rather than detail: emotional themes, deep bonds with particular people, unfinished lessons, talents, and the felt residue of how earlier lives ended. What is said to be forgotten is usually the specifics, the names, dates, faces, and the running narrative of a single lifetime, so that each new life can be entered fresh.
Different lineages fill this in differently. Some describe a life-between-lives stage where a soul reviews what has passed. Some speak of karma as a kind of moral momentum that persists. Others frame it more loosely, as patterns the person is here to work through again. These are interpretive teachings. They are meaningful to those who hold them, and they conflict with one another, which is itself a clue that they are belief rather than established fact.
In a regression session, a person in a relaxed, inwardly focused state may report a sense of carrying something across, often a feeling of recognition with a stranger, or a fear that seems older than this life. That experience can be vivid and moving. It is best understood as the mind generating imagery and emotion, drawing on memory, expectation, and imagination, rather than as retrieved data from a previous existence.
People who find these ideas comforting often use them as a lens for meaning. Framing a stubborn fear as an old lesson, or a strong bond as a soul connection, can make experience feel less random. Held lightly, as metaphor and worldview, that can be genuinely steadying.
What the frame cannot do is verify itself, and it is not a substitute for care when someone is struggling. For grief, identity questions, or persistent distress, support from people trained to help with those things matters. The teachings about what crosses between lives are a matter of faith and interpretation, and they are most honest when described that way.