Learner's definition of PIPER
a person who plays a pipe or the bagpipes
pay the piper
to pay the cost of something
We have to do what they say because they're the ones who are paying the piper.
You know what they say: he who pays the piper calls the tune. [=the person who pays for something controls how it is done]
to pay money or suffer in some way because of something you have done