When Socrates and Euthyphro meet, Socrates clarifies for Euthyphro the charges that the state has brought against him and Euthyphro is disturbed to hear about the trouble of his friend. He says that he too has been involved in a rather unpleasant set of charges, namely his own accusation against his father.
Socrates is quite surprised to hear this because in ancient Greece it was considered very bold to officially accuse one's own family member of anything, and mortals who did such were not looked upon kindly by the Greek Gods. Euthyphro admits that he is prosecuting his father for the murder of a servant and consequently, he is considered by his fellow citizens and statesman to be acting "impiously". Euthyphro, rather arrogantly, asserts that the people know not what impiety truly is, for if they did they would not consider his actions to be of the sort. This assertion indirectly indicates to Socrates that Euthyphro has knowledge of piety and impiety, and Socrates draws and analogy between his own case and the case of Euthyphro. If Euthyphro can explain to Socrates the meaning of impiety, perhaps Socrates can argue better against his own charges and so he asks Euthyphro to kindly teach him about piety, thus assuming the role not of the teacher, but of the student.
"Tell me then, what is the pious, and what the impious, do you say?", says Socrates. And the first answer Euthyphro gives is that it is to prosecute the wrongdoer, as he is doing in the case of his father. Socrates rightly points out that Euthyphro gave him an example rather than a definition and asks him to clarify his answer in the form of a definition. This bit of dialogue is important because it introduces the idea behind much of Plato's writing insofar as there is an instance here of the "one over many principle" which constitutes the theory of Plato's Forms. Again, Euthyphro has given and example of piety (of which there are many other examples) when instead Socrates wants to know what piety actually is (of which there can be only one definition).