A "useful idiot" is a derogatory term for someone who, without realizing it, advances the goals of another person, group, ideology, or movement by spreading propaganda, defending harmful policies, or legitimizing destructive ideas. Not because they are consciously aligned with the manipulators, but because they have been deceived, flattered, or ideologically captured.
The person isn't knowingly malicious. They think they are acting in line with their own values. They are usually motivated by a desire to appear moral, enlightened, or part of a higher cause. That makes them especially exploitable.
Ultimately, they are a pawn. Someone whose naivety, stupidity, vanity, or misplaced trust makes them serve ends they don't actually understand (but often pretend to).