In its neutral meaning, it simply means unaware of something, not knowing something. e.g. "He drove too fast because he was ignorant of the traffic laws of the country." (note "ignorant + of + something"). The same word has a negative connotation which is used to describe a person who is rude or does stupid things: e.g. "He keeps upsetting people with his silly comments - he is so ignorant."

Ignorant of something means that you are hardly aware of that particular thing or it's cause. It may also mean that you lack knowledge or information about it.