In the film, the fictional African country of Wakanda is led by King T’Challa (Chadwick Boseman) and populated mostly by Africans. Everett Ross (Martin Freeman), a U.S. State Department employee stationed in Wakanda who becomes an ally to T’Challa, is one of the few exceptions. Ross' presence is used as the foil turn an old African stereotype around.
When Ross attempts to speak to the leader of the Jabari tribe — M’Baku (Winston Duke) — he's told to keep quiet or M’Baku "will feed him to his children." After giving Ross enough time to internally freak out about the prospect of being eaten by other humans, M’Baku lets him in on the joke: The Jabari tribe is the opposite of cannibals; they're vegetarians.
Though the quip could easily be overlooked, it cleverly dismissed the common portrayal of people of colour as cannibals. The stereotype within pop culture dates back centuries, becoming the most prominent in the '70s. In Herman Melville's 1846 book Typee: A Peep at Polynesian Life, the Polynesian island Nuku Hiva's population is depicted as cannibals. By the '70s, the trope became so popular in movies to the point that "cannibal films" needed its own genre of exploitation films.
Cannibal films generally centered around Asian or South American tribes, using gore to shock the audience. The genre's popularity peaked from 1977 to 1981, "a period that has come to be known as the cannibal boom," IMDb explains.
The boom may have ended a few decades ago, but more recent films such as Indiana Jones and the Temple of Doom have depicted people of colour eating monkey brains and other "savage" dishes. But Black Panther's use of the trope to scare Ross beautifully disregarded it.