Very, very bad in quality. The poor piece is the “low quality” meaning, not the “no money” meaning, though those two sit near each other in English. Piss here is not about urine; it is a rude booster that makes the complaint louder and dirtier. A piss-poor plan barely works. A piss-poor excuse is one nobody believes. A piss-poor film is just a bad film. Adults use it when they are angry, or joking-angry, in British and American talk. You will not hear it in a textbook, a job email, or a speech to a teacher. Though actually, English has a few other piss- compounds that work as boosters, but piss-poor is the one you will meet most. If you need a clean word, say terrible, really bad, or very low quality.