Nyo Boto's an old midwife from Juffure. And she's also Kunta's favorite person in the universe when he's a little whippersnapper. (Aww.)
In addition, she also shows us what makes the American institution of slavery so different than other forms of slavery that have been practiced throughout human history.
This boisterous baldie is the best. Like his peers, Kunta loves to drop by her hut and listen to stories, but their relationship is closer than most. For instance Nyo Boto helps Kunta make a saphie, which is a piece of religious iconography. Although their relationship's forced to end once Kunta becomes a man, he carries the lessons she teaches him everywhere he goes.
She's also known for being bold and outspoken. In fact, Nyo Boto is famous for "her noisy defiance of the ancient tradition that decreed women should always show men the utmost of respect" and frequently talks trash to dudes as they walk past her hut (11.6).
It's pretty awesome.
Based on this, you can see why Kunta "almost swallowed a mouthful of palm fruit" when Omoro tells him that "'Grandmother Nyo Boto [...] is a slave'" (16.21). She's so bold and brash and uncompromising that he never would have even considered the idea that someone owns her. That doesn't compute.
Unlike in America, where slaves are seen as inherently sub-human and treated as commodities rather than living beings, the system of slavery in Juffure doesn't deny one's personhood, nor their right to be a part of a larger community. That's not to say that it's good or anything (owning people is the lamest thing you can do) but simply that these are distinctions worth noting.