What Neptune does in a chart
Neptune is the part of the chart that answers one question:Where do I dissolve the boundaries? It describes imagination and idealism — 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 Neptune and you get dissolves into pisces themes — porous, imaginative, merging, applied specifically to imagination and idealism.
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.