When you're informed, you have all the information or knowledge that you need. An informed group of voters has been educated in detail about all the issues and candidates.
Informed students are more engaged and excited about their classes, and of course they do better on tests. An informed decision is one that you make after learning about your options and giving the matter careful thought. Informed is from the verb inform, "give someone facts or information," from the Latin informare, which literally means "to form," and is figuratively used to mean "to educate."