FREE JAZZ
Although jazz is considered difficult to define, in part because it contains many subgenres, improvisation is one of its defining elements. The centrality of improvisation is attributed to the influence of earlier forms of music such as blues, a form of folk music which arose in part from the work songs and field hollers of African-American slaves on plantations. These work songs were commonly structured around a repetitive call-and-response pattern, but early blues was also improvisational.
Articles for your reading leisure :)
Explainer: the history of jazz
What Is Free Jazz? - 2022 - MasterClass
Free Jazz: A Short History Of The Jazz Sub-Genre
Free Jazz: The Artists and the Music of Free Jazz- Mosaic Records