Jungian Archetypes + the Collective Unconscious
One way to understand Tarot is as a symbolic mirror.
Imagine someone asking whether they should leave a situation that has become stagnant and drawing The Fool. Its imagery of departure, uncertainty and a new beginning may bring forward feelings or possibilities that the person was already carrying without having clearly expressed them.
From a Jungian perspective, symbols can interact with material beneath conscious awareness and make it easier to recognise. Jung himself connected Tarot with archetypal processes and transformation, although he never developed a complete theory of Tarot. Later authors expanded the relationship between Jungian psychology and divination considerably further.
The cards help reveal something already present within the psyche.