Douglass's narrative is, as the title page tells us, "Written By Himself." He's the book's main character – almost the only character – so most of the narrative is just him talking to us about himself. Simple, right?
Well, that's not quite all there is to it. Even though Douglass is the book's narrator, he is a bit older than he was when he was living the events he's telling us about. Sometimes he looks back on his younger self with a bit of nostalgia. Since most of the action takes place when Douglass is a teenager, he's got a slightly different perspective on the kind of a person he was when he was growing up, and he often emphasizes the things he didn't know then, or things he would have done differently.