The story is told from Cassie's point-of-view. So we only know what Cassie knows at any given time—which isn't always much, since she's only nine years old.
A story told by a nine-year-old could get a little limiting, so Taylor uses cheats—er, uses various literary techniques—to get around it. For example, Cassie hears a lot of gossip that provides readers with important tidbits of information (naughty Cassie—she is even known to eavesdrop on conversations). She also gets lectures from authority figures, such as Mama, Papa, and Uncle Hammer that give us insight into the book's various messages.
Here's one example of this at work in the novel. Cassie overhears the adults talking at church about how the Berrys were attacked:
The boys and I sat at our study table pretending not to listen, but listening still.
"Henrietta Toggins," said Mrs. Lanier, "you know, Clara Davis' sister that live up there in Strawberry? Well, she's kin to the Berrys and she was with John Henry and Beacon when the trouble got started. [...] They was waitin' there for they gas when some white men come up messin' with them—been drinkin', you know. And Henrietta heard 'em say, 'That's the nigger Sallie Ann said was flirtin' with her." And when she heard that, she said to John Henry, 'Let's get on outa here." (2.54-5)
So, you might ask, why does Taylor have Cassie narrate the story in the first place, since she comes with all these limitations that have to be worked around? For starters, it allows readers to have the experience of learning along with Cassie. It also allows us to see her childish innocence, even though she's subjected to some pretty harsh reality. And, of course, it's a novel written for kids or young adults—so, having a young narrator makes it easier to read.