Legal ignorance refers to the lack of knowledge about the law or legal rules, which can sometimes be used as a defense in criminal cases. This concept is important because it emphasizes that individuals may not always be aware of the laws they are breaking, and in certain situations, this lack of knowledge may affect the outcome of their legal responsibility. However, the principle of 'ignorance of the law is no excuse' often prevails, indicating that individuals are generally expected to know the law.