In astrological tradition, the Sun governs core identity and the sense of self, while Pisces is a water sign read as empathic, imaginative, gentle, and boundary-soft.
So Sun in Pisces is often read as a placement where how someone shines and seeks to be recognized takes on a empathic, imaginative quality. Tradition holds that someone with this placement tends to expresses its vitality in a way that feels gentle — coloured by the water-sign temperament of Pisces rather than by any single rule.
None of this is a prediction or a fixed trait. These are tendencies astrologers have long associated with Sun in Pisces, and how they actually show up depends on the rest of the chart — the houses, the aspects, and the other placements that this guide cannot see.
A general guide to astrological tradition — tendencies, not predictions. It describes the placement in the abstract and is not about any specific person.