To be confirmed is to be, undeniably, an anti-town role. While similar to clear in context, the two words are used separately to differentiate good and bad. For example, if Bob is undeniably a town role, he is clear, but if Bob is undeniably an anti-town role (like mafia), then he is confirmed.
Occasionally, users use the term loosely, calling someone confirmed (or clear) based on reasons other than undeniable logic (such as meta or reads (add link)).
In common instances, being confirmed typically results in an immediate lynch, but there are situations in which lynching a confirmed role is not the best idea, depending on the setup.