What is the meaning of ignorance of the law?

In an earthly court of justice, it counts for nothing that you are ignorant of the law, and through that ignorance, transgressed it. The court exacts the penalty just the same, holding you responsible for your ignorance, saying you should have informed yourself―

We now say to you―

Natural law requires you to pay the penalty of transgressed law whether it be transgressed ignorantly or otherwise, thereby compelling you in a measure to inform yourself.

―Spirit Robert G. Ingersoll