Mabel, improvisation in music is a huge topic which I can conduct an hour-long talk on it. To summarise, it is to make variations to the original music (instead of adhering strictly to its original score) via a few means. For example, one way of improvisation is to substitute some of the chords used (or even adding new chords in between existing chords). Another way is to play out the chords differently (e.g. from broken chord style to rhythmic style). Adding or changing other textures/layers such as counter melodies and bass lines also constitute improvisation.

Some singers or solo instrumentalists improvise on the melody by changing some of the notes occasionally. Jazz improvisation, on the other hand, allows complete composition of melodic lines along the original music's chord progression.

More advanced improvisations can include addition of new sections (such as adding/replacing an original interlude), employing modulations and change in rhythm/time signature.