Symbolism, Imagery, Allegory
Grandma's basically do two main things in literature: cook killer grub and teach their grandkids lessons. Grandmother Yaisa's cooking skills aren't really noted in Roots, but she does tells Kunta a little parable that has a big impact.
It's about a little boy who helps a crocodile that's stuck in a net. After the boy frees it, the crocodile immediately clamps its jaws around his body and threatens to eat him. (Whoa: is this Just Say No: Crocodile Edition?)
And then this exchange happens:
"Is this how you repay my goodness—with badness?" cried the boy.
"Of course," said the crocodile out of the corner of his mouth. "That is the way of the world." (3.15-16)
Eventually, the boy's freed by a rabbit. And should be thankful, right? Well, he ends up telling his family what happened, and they return to kill both the croc and the rabbit. And now the poor kid has just done exactly what he accused the crocodile of doing—he repaid goodness with badness.
As it happens, this lesson gets taught again and again throughout the novel. Kunta's service to Massa Waller is repaid with him selling his daughter. Kizzy's friendship with Missy Anne is repaid with betrayal. Even Chicken George, who is undyingly committed to Massa Lea, learns that his owner is unwilling to pay that loyalty back in kind. The list goes on and on.
It might not be the most heart-warming allegory in the world, but that's what makes it such an important one. Plus, it definitely hammers home the point that crocodiles are not to be messed with.