A turnaround is a kind of musical cadence, a progression of chords that provides harmonic "punctuation" in a composition. Turnarounds are used to establish a tonal center (or key), to mark the end of a section of a tune, to return to the beginning of the previous section or to prepare for the beginning of a new section. A turnaround can also serve as an independent repeating section of a tune, often occurring in the form of a coda.
The harmonic material of many standard tunes consists of nothing more than variations of turnarounds. They can be thought of as essential formulas for understanding how compositions from the mainstream jazz and popular repertoire "work" harmonically. Though there are many jazz composers who've extended the language far beyond these harmonic formulas, the ability to understand and make creative use of turnarounds is essential for the jazz improvisor.