Critic speak is tough, but we've got you covered.
Quote :"Letter to Margaret Harkness" (1888)
The more the opinions of the author remain hidden, the better for the work of art. The realism I allude to may even crop out even in spite of the author's opinions. [...] That Balzac thus was compelled to go against his own class sympathies and political prejudices, that he saw the necessity of the downfall of his favourite nobles, and described them as people deserving no better fate; and that he saw the real men of the future where, for the time being, they alone were to be found—that I consider one of the greatest triumphs of Realism, and one of the grandest features in old Balzac.
Good ol' Balzac. For those without fond memories of the guy, he was a great French realist novelist. He loved the upper classes, and wasn't so fond of the lower orders, but according to Engels, Balzac was such a great realist because despite his own interests in prejudices, he couldn't help recreating history in his novels—even when that history went against his own ideology.
For Engels, it's not about how talented you are with words or how unique your perspective is or how well you understand your characters—it's about how accurately you describe reality. (Marxist reality, that is.)