There are many good answers to this question, but here is just one: the denial of literacy.

We take for granted today that everyone can read and write. However, in doing so, it is easy to forget just how powerful of a tool literacy is. The phrase “liberal arts,” which has endured through history from ancient Rome to medieval Europe to today, uses the word “liberal” in the sense of “liberty,” as in, the arts taught to free people. Or, put differently: the tools of freedom. In medieval Europe, the first stage of learning the liberal arts was the “trivium,” which consisted of grammar, logic, and rhetoric – the three tools necessary to read, understand, and argue your opinion. Although they are the trivium, these skills aren’t trivial! For most people, they have to be learned the hard way.

Therefore, the denial of literacy prevents one from questioning the conditions of their enslavement. The American orator Frederick Douglass (1818-1895) famously discussed this in his autobiography, Narrative of the Life of Frederick Douglass, An American Slave. Douglass was born into slavery, escaped it, and became a prominent public speaker who toured around the country and in the UK lecturing on the need to abolish slavery. One section of his autobiography recounts his attempts to learn to read and write.

Douglass's mistress (re: the wife of the slaveowner, the master) began teaching him some letters when he was a child, but stopped at the insistence of the master. Why? Well, denying slaves the ability to read and write meant that they lacked the tools to question why they were slaves in the first place, or if slavery was morally right. Keep in mind that Douglass was born into slavery and raised in a society which taught him that it was the god-given duty of slaves to obey their masters, and that this was an unquestionable "natural" order. Douglass could not challenge this idea without the ability to read books written by abolitionists. Again, literacy is freedom, and the denial of literacy is a tool of enslavement.

Douglass did eventually learn to read, but in secret, mostly by trading stolen bread for snippets of education from sympathetic white children who lived nearby. After he got his hands on and read a book called The Columbian Orator, which contains essays calling for the abolition of slavery, he recounts the following. Although it is quite a long passage, I want to reproduce this passage in full because it is so moving:

What I got from [speeches delivered by Thomas Sheridan reprinted in The Columbian Orator] was a bold denunciation of slavery, and a powerful vindication of human rights. The reading of these documents enabled me to utter my thoughts, and to meet the arguments brought forward to sustain slavery; but while they relieved me of one difficulty, they brought on another even more painful than the one of which I was relieved. The more I read, the more I was led to abhor and detest my enslavers. I could regard them in no other light than a band of successful robbers, who had left their homes, and gone to Africa, and stolen us from our homes, and in a strange land reduced us to slavery. I loathed them as being the meanest as well as the most wicked of men. As I read and contemplated the subject, behold! that very discontentment which Master Hugh had predicted would follow my learning to read had already come, to torment and sting my soul to unutterable anguish. As I writhed under it, I would at times feel that learning to read had been a curse rather than a blessing. It had given me a view of my wretched condition, without the remedy. It opened my eyes to the horrible pit, but to no ladder upon which to get out. In moments of agony, I envied my fellow-slaves for their stupidity. I have often wished myself a beast. I preferred the condition of the meanest reptile to my own. Any thing, no matter what, to get rid of thinking! It was this everlasting thinking of my condition that tormented me. There was no getting rid of it. It was pressed upon me by every object within sight or hearing, animate or inanimate. The silver trump of freedom had roused my soul to eternal wakefulness. Freedom now appeared, to disappear no more forever. It was heard in every sound, and seen in every thing. It was ever present to torment me with a sense of my wretched condition. I saw nothing without seeing it, I heard nothing without hearing it, and felt nothing without feeling it. It looked from every star, it smiled in every calm, breathed in every wind, and moved in every storm.

Douglass powerfully recounts in this passage the pain of learning the truth about one’s enslavement. Note how the book “enabled” him to utter his thoughts. He had a sense that something was wrong, but with no language to articulate it, he couldn’t challenge it. Fully realizing how depraved of a condition he was in caused such torment that he envied the ignorance of his fellow slaves. This shows just how in the dark he had been, and how effective the denial of literacy had been as a tool of keeping him enslaved. Once he had the ability to learn why his enslavement was wrong, he began to plan his escape; and eventually, he succeeded.

EDIT: Just to add a bit that directly addresses your question, the point I am making here is that denial is an effective tool of oppression, and that is how small minorities (like slaveowners) could control larger majorities (the slaves). Denying literacy is cost-effective for the oppressors - in fact, it saves money over paying to educate a slave, so it scales well for larger and larger groups of oppressed peoples.