What does 'ignorance of the law is no excuse' mean?

This classic saying means that you can not generally defend yourself against a claim on the grounds that what you did, or failed to do, was not legal. While it often pertains to criminal law matters, it can pertain to others such as tax, administrative, etc. The underlying reasoning is that in any kind of orderly society the citizens must be held to account to know what the law is, and if not known, to find out what it is first before acting.

An example is a car speeding through a suburban residential street at 50 mph. The driver defends on the grounds that there was "no speed sign". The driver will be held to know that the general rule of law is 30 mph and if not sure then 50 mph certainly is inappropriate.