Hey there! I'm happy to help!
Justice is holding people accountable for their actions and treating them accordingly. If you live in a home with many rules but no punishments for breaking them, hardly anybody will keep the rules because there will be no consequences!
The same goes for the government. If there is a law against robbery but there is no punishment for robbery, then the law is no good! People will do it anyway if you do not hold them accountable. There has to be an "or else" to encourage people not to do it. High crimes like murder must have punishment so that people do not do it. If the penalty for murder is execution, hardly anybody will commit murder because it will cost them their life.
Establishing justice is laying out how punishments and reprimands will be given to those who break the law so that fewer people break the law and criminals receive the consequences they deserve, leading to a safer and better nation.
I hope that this helps! Have a wonderful day! :D