Improvising is when a performer makes up the music they are playing on the spot! It is an important feature in jazz music when musicians improvise around known melodies or chord patterns.
North Indian classical musicians improvise on a raga, whilst church organists often improvise whilst waiting for services to begin. Many of us improvise subconsciously when we hum or sing in the bath!
Composers might use improvisation as a first step in the composition process, to explore and develop their musical ideas, maybe you could try doing this too?