What Sun does in a chart
Sun is the part of the chart that answers one question:Who am I when I feel most like myself? It describes core identity — not events, and not outcomes.
What Pisces does to it
Pisces is a water sign, so it moves on feeling and atmosphere. It is also mutable, which means it adapts things. Put those together with Sun and you get identity through imagination — porous, imaginative, merging, applied specifically to core identity.
Pisces is ruled by Neptune, so this placement tends to be coloured by imagination and idealism as well.
The useful version
Placements are not verdicts. This one describes a default setting, not a limit — the things listed above are what you do when you are not thinking about it, which is exactly why noticing them is worth something.