A Representation represents, or is about, a certain object or state of affairs (the representation's object) and says something about that object (the representation's content). Misrepresentation happens when what that content says about the object isn't true of the object. For instance my cow representation has a certain content; suppose that this content is something like that's a four-legged mammal that gives milk, goes "moo", and eats grass. Anything this representation "is about" will be represented as something that description applies to. So if my cow representation is activated by--and thus refers to--a short fat muddy horse seen from a distance, that horse is misrepresented, because it's represented as a four-legged mammal that gives milk, goes "moo", and eats grass, which is false of the horse.
Theories of content, which attempt to explain how representations correctly represent their objects have a tremendous amount of trouble explaining how they can also sometimes misrepresent their objects. Jerry Fodor's (1990) disjunction problem points out the difficulty here. A representation's content can't be such that the representation represents whatever causes its activation. A representation with content construed in this way can't misrepresent.