It means believing that all people who commit crimes should be punished, regardless of other circumstances.
ex. If a single father of 2 stuck in a life of poverty had to steal from a grocery store to feed his children, you believe he still deserves to be punished to the full extent of the law. If you valued mercy, you would believe that he deserves a light sentence, or no punishment at all because it was a crime out of necessity.
Suppose a man with starving kids robs a grocery store and kills a clerk that is a single mom with two kids. Valuing justice higher would always punish the man for violation of the law and the harm he did to the family of the clerk. Valuing mercy higher would let him go at the expense of the family. This is an great example of a moral dilemna.