The treble crochet, sometimes called triple crochet, (abbreviated tr) is one of the basic stitches. It is a little taller than the double crochet (dc). This stitch creates a loose fabric.
How to make the treble crochet (tr)
Yarn over your hook twice, insert hook in the specified stitch, yarn over, pull through the stitch, yarn over again and pull through 2 loops, yarn over again and pull through the next 2 loops, yarn over again and pull through the last 2 loops on the hook.
The treble crochet (tr) is referred to as double treble crochet (dtr) in patterns using British terminology. Here you can find a helpful US to UK conversion chart.