I have heard people say so many times “when jazz people improvise, they make things up as they go along.” Even some music appreciation textbooks that I’ve seen say pretty much the same thing.
Nothing could be further from the truth.
Think about it. Very few jazz performers work completely alone. To “make things up as they go along” would only invite chaos. Instead, every player in the group improvises on the underlying structure of each piece of music played, sharing an understanding of this with all the other musicians in the group.
In fact, a huge body of jazz depends on working with what are called “standards.” These can be blues like “St. Louis Blues,” or songs like “I Got Rhythm,” “Body and Soul,” or “Summertime.” All of this music has a definite form and phrase structure as well as a pattern of chords that serve as the underlying skeleton upon which the improvisation is built.
In most live performances of jazz, the reason the audience applauds after various musicians’ solos–that is, their improvisations on the bones of the original piece–is that a good improvisation is such a source of real satisfaction, indeed happiness, to both performers and to any perceptive listener as we hear a new “take” on an old melody.