The full proverb is "He who pays the piper calls the tune." This old saying means, simply, that if you are paying for something (or making yourself responsible for it in some way) then you get to decide how it will be done; just as, if you are paying a musician to play for you, you would expect to be allowed to choose what music you like.
In a way this is similar to the more modern sayings, "The customer is always right" and "the customer is king." However, these sayings are more usually uttered if there is a strong sense that the customer may, really, be wrong. ("I'm sure that dress isn't going to suit her, but she insisted on buying it, and the customer is always right," a shop assistant might say. The idea is that you don't disagree with your customers, even if you think they are making a mistake; whereas the "piper" saying is more about reminding consumers that if you are paying for something, you have a right to choose it.
In a way this is similar to the more modern sayings, "The customer is always right" and "the customer is king." However, these sayings are more usually uttered if there is a strong sense that the customer may, really, be wrong. ("I'm sure that dress isn't going to suit her, but she insisted on buying it, and the customer is always right," a shop assistant might say. The idea is that you don't disagree with your customers, even if you think they are making a mistake; whereas the "piper" saying is more about reminding consumers that if you are paying for something, you have a right to choose it.