No crime without law meaning

To mount a prosecution against a person in the absence of domestic criminal provisions applicable at the time of commission of the criminal offense, the definitions of the criminal offenses of genocide, crimes against humanity, and war crimes must therefore be sourced from customary international law.

There is to be no penalty without written law.
That is, criminal prohibitions must be set out in written legal instruments of general application, normally statutes, adopted in the form required by constitutional law.
This excludes customary law as a basis of criminal punishment.
There is to be no penalty without well-defined law.

Applying both of these concepts, “no crime without law, no punishment without law” means that if a law is not in place to prohibit an act at the time the act is committed, then the act cannot be considered criminal, nor be punished, even if legislation is later passed that criminalizes the act in question.