Someone well-informed knows a lot — either about one subject, or about the world in general. Two shades sit inside it. First, general: a well-informed person reads, follows the news, can talk about most things without saying something silly. Second, specific: well-informed about something in particular — "she is well-informed about labour law", "he keeps himself well-informed on climate policy". Both are compliments, and both praise the habit of finding things out rather than raw cleverness. Nobody is born well-informed; you become it by paying attention, which is exactly why teachers and journalists like the word. There is a third, narrower use that is worth knowing because it appears constantly in news writing: a well-informed source is a person who has real inside knowledge of something, usually someone close to a government or a company who is telling a reporter what happened. In that phrase the word means "in a position to know", not "well-read". It works before a noun ("a well-informed audience") and after be ("they were surprisingly well-informed"), and it takes more/most rather than -er/-est.