What does the phrase pay the piper mean?

That's from the Pied Piper. In the tale, the Pied Piper was called in to rid rats from the town, which he did by playing his pipes and luring them to their death by drowning. When he returned to the town for payment, he was not paid, and he took it out on the townspeople by again playing his pipes, this time luring all of the children out of the town.

So, to "pay the piper" means to face the inevitable consequences of one's actions.[1]