They want to prevent him from reading.
This passage is in the section of the memoir when Douglass is talking about how they wanted to keep him from an education. The paragraph this text is found in begins "Mrs. Auld was an apt woman, and the advice of her husband, and her own experience, soon demonstrated, to her entire satisfaction, that education and slavery are incompatible with each other." It goes on to say how he already knew his alphabet so her concerns about him getting an education were too late. He already had the knowledge he needed to continue his studies.
