I was going to make a cheesy Lord of the Rings allusion and say "one line to rule them all."  Luckily, I didn't. Oh wait. I did. Too late now. But it's not too late to come up with a great one-liner for your play.  A one-liner is part organizational tool, part sales pitch--the idea is to give us a sense of who's in the play and what the conflict/problem will be, without giving away the ending.  Make it active.  For example:

Two teen lovers must overcome the enmity of their families to be together.  (Can you figure out what play that might be?)

We know now that the play will be about a pair of teenage lovers and their battle to be together, despite the feud between their families.  It's not to say some other things won't happen in the play, but the one-liner helps you "center" the play around this dramatic action, to make sure that this central story keeps driving forward. Having trouble formulating a one-liner?  It probably means that you haven't quite figured out what the play is about yet.  Happy writing!