Justice is the quality of being just or fair. Super heroes fight for justice because they want everything to be fair and not let bad guys win. Just ask Wonder Woman, Superman, or any other member of the Justice League.
Justice is usually associated with the law. A judge is also known as a justice, and the point of the law is to keep everything in a society fair according to society’s rules. Justice (capital “J”) is also the statue of a blindfolded woman holding scales and a sword. If something is brought to justice, the good guys have been rewarded and the bad guys punished — the scales are even.