The full title of this book is Narrative of the Life of Frederick Douglass, An American Slave. Written by Himself. Pretty straightforward title, right? Frederick Douglass was a slave, and this is going to be the narrative, or story, of his life. Well, that's what it is, and sometimes straightforward is good.
But there are also a couple things worth noticing here. First, by calling himself an American slave, Douglass is reminding his audience that slavery didn't happen in some faraway land; it happened in America, the "land of the free." We're supposed to notice that and think about it. Douglass loves this kind of irony, and reminding us that the land of the free is also the land of slavery is just the kind of hypocrisy he likes to point out. (Years later he gave a famous speech entitled "What to the Slave is the Fourth of July?" which you can read here.)
The second thing is the phrase "Written by Himself." That might seem basic, too, but there's a serious point there. Most white Americans in the mid-1880s had a hard time believing that a black person could even learn to read, much less write a book. In fact, when the book became a bestseller, there were a lot of skeptics who insisted that he must have had help. This is why Douglass included two different prefaces from famous writers at the beginning, to sort of vouch for the fact that he did it all on his own. Just the fact that he wrote the book by himself was a way of proving that black people were the equal of whites.