The title Rhythm Changes refers to a progression based on the George Gershwin song I Got Rhythm. Over the years there have been many new melodies (or heads in Jazz terms) written on this progression and it is still routinely used by musicians jamming together or auditioning a new band member.
This example shows the basic progression in the key of B Flat. On the recording it is played without a melody. Try improvising along with the recording, using your knowledge of chords and keys, but also relying on your ear. This recording is quite fast (mm=220), so if you have trouble with it, practice with some slower songs first. In time, faster tempos will become easier. Ella Fitzgerald was amazing at improvising vocal solos at fast tempos.