Under "Tools" click "Predict biological activities". You need to have a structure displayed in the drawing window.
PASS (www.akosgmbh.de/pass) predicts biological effects for drug-like compounds. The predictions are at least 80% accurate. Pa = 0,996 means that PASS predicts that taxol has with 99.6 % probability the effect "respiratory analeptic". Pi is the probability of having not this effect. Pi is calculated independently from Pa.