A turnaround is a chord progression that occurs at the end of a tune. It is designed to get you back to the beginning, to "turn it around". Usually, it is a 2-bar progression consisting of four chords, two beats each.
The most basic is the so-called 1-6-2-5 turnaround. In the key of C, it would look something like this:
We can replace the Cmaj7 (the I chord) with Emin7 (the III chord) and the turnaround still functions the same way:
The 2nd chord of the turnaround doesn't have to me a min7. It can be an "altered" 7 chord. We can also add an augmented 5th to the last chord for extra "flavor":
Here's a completely different turnaround taken from the Miles Davis tune "Half Nelson". It makes use of the "tritone substitution" principle (more on that later):
Listen to all the turnarounds in a row:
You should learn these in all keys. These turnarounds should be a permanent part of your playing. They happen at the end of every chorus of a tune, so you're gonna play a lot of them from now on!