Final answer:
Henry IV meant that he was willing to convert to Catholicism to become the King of France and bring stability to the country.
Step-by-step explanation:
When Henry IV said, 'Paris is well worth a mass,' he meant that he was willing to convert to Catholicism in order to gain more support and ultimately become the King of France. At the time, France was largely Catholic and the majority of the population expected their ruler to be Catholic as well. By saying this, Henry IV was emphasizing the importance of religion and demonstrating his commitment to the Catholic Church. This decision helped bring stability to the country and allowed for more religious tolerance.