Answer:
By forbiding his wife to teach Frederick how to read, Mr. Auld makes it clear for Douglass that the only thing that kept black man as slaves was their ignorance. White men made sure that black people did not have access to an education, because if they were to be educated, they would attain a voice of their own.
Douglas finally understood that which he had been strugglin with for years. Through his master's words, he knew that the only way to be free would be through knowledge.
extra answer Douglass learns the white man's power comes from the ability to prevent education to the slaves.
The excerpt expresses Douglass' understanding of how education empowers the literate person and by remaining illiterate, unable to read, the master keeps his power against the slave.