What is Ignorantia Facti Exensat, Ignorantia Juris Non Exensat?

Legal Definition
Ignorance of the fact excuses; ignorance of the law excuses not. Every man must be taken to be cognizant of the law; otherwise there is no saying to what extent the excuse of ignorance may not be carried. 1 Coke, 177; Broom, Max. 253.
-- Black's Law Dictionary