Red Sonja is right. Everyone defines themselves differently, and everyone has different interpretations of each title. But pretty much everyone agrees that there are two sides to BDSM - a top person who is dominant and a bottom person who is submissive. PYL refers to any type of top person, and pyl refers to any kind of bottom person. These are not typically used in a sense of "me and my PYL got together one night" - usually a person knows how their own partner identifies, so you'd just say "me and my Master got together one night" and leave it at that. If you aren't sure what title somebody uses, though, like "how are you and your sub/slave/puppy/toy/bottom doing today" then the abbreviation comes in handy - "how are you and your pyl doing today?"
Hope that helps, and feel free to ask about any other abbreviations or terms you don't understand!