Turnarounds are one of the most important concepts in jazz (and music in general) because they are what binds the musical sections together. Their job is to lead the listener to the next section while still preserving their whole attention and excitement. Especially if you are going to repeat the same section one more time, turnarounds may be the key to not lose your audience.
There are many ways to perform a turnaround. You can make it a harmonic turnaround with a different chord progression (we'll look into some of them used in jazz) or a melodic turnaround that suits your own needs.
Of course, some of the most well-known turnarounds in jazz history comprise of a ii-V-I progression hidden inside it, like a I-vi-ii-V or, for a chromatic bass line, a I-VI-ii-V that leads back to the tonic chord. A typical turnaround starts with I (tonic) and ends with a V (dominant). But by the power granted to jazz musicians, you can use dominant substitutes like bII7 to create your own unique turnarounds.
A great example of this is the Tadd Dameron turnaround, named after the great jazz composer/pianist. It was first used in Dameron's jazz standard "Lady Bird" and is actually derived from a typical I-vi-ii-V we talked about just now; but using the tritone substitutions (we'll make another post about them too, stay tuned) of the vi, ii and V chords, you get the Dameron turnaround: I-bIII-bVI-bII7. The piece also contains a modulation down a major third which might have been the precursor to the "Coltrane changes".