PnP stands for "party and play." In this context, to "party" means to take illegal drugs, typically meth, and to "play" means to have sex, often referring to gay and/or group sex. Both PnP and party and play are euphemisms or code words that allow people looking for PnP partners or participants to openly invite, discuss, and organize PnP events in relative secrecy, while potentially being overheard by others who might judge, berate or interfere with PnP planning.
For someone to find partners with whom they can take drugs and have sex, they need to be able to invite strangers to take part in two highly stigmatized behaviors: taking an illegal drug, and having sex with someone they don't know. Stigma is heightened by the sexual activities in question -- namely gay sex, particularly anal sex and/or unprotected sex -- also known as barebacking -- and group sex. One of the ways that people interested with finding people to take drugs and have sex with is through online chat rooms and dating sites, where terms like party and play and PnP allows people to invite others without explicitly mentioning drugs or sex.
However, as the terms party and play and PnP have become more widely recognized, it is becoming more difficult to use these terms without detection, and websites may not allow use of these terms. Certainly, if you use the terms party and play or PnP in online chat rooms or dating sites or gay clubs, many people will assume you are referring to taking meth before having sex.
PnP is appealing to people with combined meth and sex addiction, although others attempt to PnP on weekends, or only occasionally. Either way, PnP is a very risky activity, which is why online chat rooms and dating sites discourage it.