Ignorantia juris non excusat is a Latin term that, loosely translated, means: "Ignorance of the law is not an excuse for breaking it." In other words, criminal and civil defendants cannot claim innocence due to not knowing a particular law. It is the burden of citizens to be aware of the laws of the land.
Under the principal of ignorantia juris non excusat, a person would not, for example, be able to rob a bank, and then claim that they did not know that robbing a bank was illegal (as a criminal defense). Trying to use ignorance as a defense would not stand up in court. In such a situation, the person would still face criminal charges and be prosecuted as if they knew that it was a crime to rob a bank.