Rate of change can be understood along the continuum between sudden change and gradual change. Rate of change in music refers to the way the music moves from one section to another. More broadly, it refers to the breadth and depth of the changes in a piece of music.
Sudden change is what happens when there is a large amount of change in a short space of time. This change can be across any musical parameters: Pitch, harmony, rhythm, density, texture, volume, etc. The bigger the change and the shorter the transition, the more sudden the effect is.
Gradual change is the opposite. This is what happens when there is a small change or a long transition. Any sudden change can be made softer by either reducing the difference between the ‘before’ and ‘after’ sections or by making the transition time longer.