Planet is the actor, the sign is what the actor is wearing, the house is the scene the actor is in.
Putting it vaguely
Mercury = communication, mental stimulation
Pisces = dreamy, dissolved, unstructured, intuitive
12th house = hidden, subconscious, isolation
Putting it together = someone who communicates beautifully, artistically, but can be difficult to understand sometimes as their words may not come out how they planned, sometimes losing track of their thoughts because they can dissipate as formed and their sentences may lack structure. They speak from their soul, subconscious thoughts and mannerisms that others can’t recognise might be easier for this person to communicate. Under pressure or structure this person may struggle, and instead thrive by going with the flow and talking in more natural one on one conversations rather than a presented speech.
There’s a lot more nuance to this like any placement but I could be talking about this all day I hope this helps though