As mentioned on the previous page, the term "Rhythm Changes" means a progression based on a song by George Gershwin called I Got Rhythm. Over the years there have been many new melodies (or "heads" in Jazz terms) written on this progression and is still routinely used by musicians jamming together or auditioning a new band member.
The following example shows the basic progression in the key of C. On the CD it is played without a melody. Try improvising with it, using your knowledge of keys and scales but also relying on your ear. The following page contains a solo played over this progression.
Here is a solo played over Rhythm Changes with substitutions at various points. On the recording it is played fast (mm 3 =220), but learn it slowly at first. Once you learn a phrase, make up your own variations on it as well as improvising over the Rhythm Changes demo.