A sequence of chords, or the portion of a tune that they occupy, that forms a cadence at the end of a section of a tune, definitively establishes the tonic key and leads back to the opening chord of the next section, or to the top. Typically the turnaround chords are I - VI - ii - V, with half a measure apiece. With possible substitutions and alterations, the variations are infinite. There are also entirely different progressions possible. If the opening chord of the next section is not a I chord, the turnaround must be suitable. Learning to negotiate turnarounds is essential to making a coherent solo. It's often effective to play a phrase that starts partway through a turnaround and continues past the beginning of the next section.