Justice is, basically, fairness. It''s the concept symbolized in the US by the blindfolded woman with the scale.
In the Job passage, Job is calling his friends out for claiming that God is unjust.
Proverbs 8 is all about a person called Lady Wisdom. Christians usually interpret this Lady Wisdom to be a representation of Logos of Jesus. "By Lady Wisdom kings reign, and princes make laws that are fair."
Proverbs 21 repeats a lesson that appears over and over in the bible. "What does the Lord require of you but to do justice and love mercy and walk humbly with your God."
You asked, "who are the princes?" You might as well ask, in the 21st century CE, "who makes decrees, or laws?" In representative democracies like the USA, it's elected legislators from town selectmen to US Senators. The Proverbs passage declares where they get their wisdom. (Note that other chapters of Proverbs declare where they get any foolishness and corruption they may have.)
Blessings on your study,
