To make something up on the spot.
Improvisation is common in jazz music, when a musician makes up a solo spontaneously, as opposed to playing a written solo.
Sometimes a group of musicians will completely improvise an entire song. This is done almost exclusively by jazz musicians.
All forms of art can be improvised. Art that is improvised may be called "free" or "freestyle", like "free poetry".
A popular form of improvisation is rapping, or flowing.
Miles Davis was a master of improvisation.