The half double crochet (abbreviated hdc) is a simple and versatile stitch. It is taller than a single crochet (sc), but shorter than a double crochet (dc).
How to make the half double crochet (hdc)
Yarn over your hook, insert hook in the specified stitch, yarn over, pull through the stitch, yarn over your hook, pull through all 3 loops on hook.
The half double crochet (hdc) is referred to as half treble crochet (htr) in patterns using British terminology. Here you can find a helpful US to UK conversion chart.