Have you ever been called upon to be a witness? When that happens someone is saying that they trust you. They trust that you will say what you know to be true and not anything more or less. Your character and reputation may have spoken to them of your consistency in doing so.
We are all walking witnesses of the moments we have succeeded (and failed) to embody what we believe (and disbelieve). When someone asks about why we act a certain way, or do a certain thing, they have entrusted us as a witness of the underlying stuff (belief, value set, worldview, etc.) that drive our choices.