Short answers, pulled from the story.
Signifyin' is a verbal practice in African-American culture and African-American Vernacular English that exploits the gap between the literal and figurative meanings of words. It uses indirection, implication, and wordplay to communicate meanings accessible only to those who share the cultural values of a given speech community. A simple example is insulting someone as a way of showing them affection.
Henry Louis Gates Jr. wrote The Signifying Monkey, published in 1988. The book argues that signifyin(g) is a trope that subsumes metaphor, metonymy, synecdoche, irony, hyperbole, litotes, and metalepsis, and that it exemplifies the distinguishing property of black discourse through double-voiced repetition and reversal.
Signifyin' is said to have originated during slavery in the United States, derived from tales about the signifying monkey, a folk trickster figure. Gates traced the practice to the trickster archetype found in African mythology, folklore, and religion.
Playing the dozens is a verbal game in which participants try to outdo one another with escalating insults, and it is recognized as a prime example of signifyin'. Tom Kochman documented an example in Rappin' and Stylin' Out: Communication in Urban Black America, published in 1972.
Gena Dagel Caponi identified signifyin' in hip hop and other African-American music through calls, cries, hollers, riffs, and overlapping antiphony. Schloss connected the looping of musical samples to signifyin(g), arguing it allows producers to use other people's music to express their own compositional ideas while obscuring the full extent of their creative agency.
Architect Scott Ruff argued that African-Americans' creation of secret pathways and clearings behind slave plantations constituted signifyin(g) in physical space. Walter J. Hood has also demonstrated signifyin(g) in his architectural works, which are described as adaptable, self-expressive, and historically-rooted spaces influenced by African-American spatial practices.