A more thorough explanation:

Definition: Ignorance of fact is an excuse; whatever is done under a mistaken impression of a material fact is excused or provides grounds for relief.

This means that if someone does something wrong because they didn't know all the facts, they might not be punished for it. For example, if someone accidentally takes something from a store without paying because they thought it was free, they might not be charged with theft because they didn't know they were supposed to pay for it.

However, this principle doesn't always apply. If someone does something that is still a crime even if they didn't know all the facts, they can still be punished. For example, if someone shoots someone else thinking it's a toy gun, they can still be charged with murder because it's still a crime to kill someone, even if they didn't know the gun was real.

Example: A man is driving down the road and sees a stop sign. He thinks it's a yield sign and doesn't come to a complete stop. A police officer pulls him over and gives him a ticket for running a stop sign. The man argues that he didn't know it was a stop sign, but the officer tells him that "ignorantia facti excusat" doesn't apply in this case because it's still a traffic violation to not come to a complete stop at a stop sign, even if the driver didn't know it was a stop sign.