Something described as being decidedly a certain way is absolutely that way, with no hint of doubt. If a character in a movie is decidedly evil, there's no question in your mind about it, it's so clearly true.
The adverb decidedly is good for providing a strong emphasis. Your mom might be decidedly unhappy about your new car, or your grades might be decidedly improving this semester. In both cases, these things are undeniably clear. Decidedly comes from the adjective decided, or "resolute," from the Latin root decidere, "to decide or determine."