JAZZ IMPROVISATION, (a partial description from Wikipedia) Basically, improvisation is composing on the spot, in which a singer or instrumentalist invents solo melodies and lines over top of a chord progression played by rhythm section instruments. Jazz improvisation is distinguished from other genres use of this approach by the high level of chordal complexity, often with one or more chord changes per bar, altered chords, extended chords, augmented chords, and ii-V-I progressions, all of which typically move through multiple keys within a single song. https://en.wikipedia.org/wiki/Jazz_improvisation
Improv, short for improvisation, in jazz is a spontaneous performance without specific or scripted preparation. In order to improvise successfully you must have a plethora of knowledge and skill to draw from. You must be familiar with the song, it's melody and chord changes. You should be armed with a supply of jazz chord substitutions. It's helpful to know various scales and modes that could be used appropriately over this song.
In this section you will learn the theories of improv as well as lessons that will not only teach you to improvise over a particular tune but that give ideas to use for improvising over other songs.