A Turnaround is simply the term for a pattern of chords used to get from one main line chord to the next.
For example: if a song's main chord pattern is a I IV V, you would use a turnaround to get from the I to the IV or from the V back to the I. All they are is filler chords to make a song sound better, but they aren't neccesarily a main part of the song.
The most basic turnaround is a ii V I used at the end of a verse before the start of the next verse or chorus. If you try some of the examples that were given, and insert them into songs you already know how to play, you'll immediately see an improvement in your playing and so will others.