Interpretation
The framing material of the parable explains that it demonstrates the need to always pray and never give up, for if even an unjust judge will eventually listen, God is much quicker to do so. The parable of the Friend at Night has a similar meaning.
Joel B. Green sees in this parable an injunction not to lose heart, in the light of the eschatological tone of Luke 17:20-37, and also an echo of Sirach 35:
For he is a God of justice, who knows no favorites. Though not unduly partial toward the weak, yet he hears the cry of the oppressed. He is not deaf to the wail of the orphan, nor to the widow when she pours out her complaint; Do not the tears that stream down her cheek cry out against him that causes them to fall? He who serves God willingly is heard; his petition reaches the heavens. The prayer of the lowly pierces the clouds; it does not rest till it reaches its goal, nor will it withdraw till the Most High responds, judges justly and affirms the right.
Read more about this topic: Parable Of The Unjust Judge