The front post double crochet stitch, or "fpdc" as it is abbreviated in patterns, is a great way to add a little texture to your crochet. It makes a nice little rib in your work (check out the photo below) and has a very dimensional texture.
Below are two swatches, the one on the right is two rows of regular double crochet stitches, and the one on the left is two rows of front post double crochet stitches. If you were to flip this swatch over, it would look like the photo above.
Let's work a stitch...begin by wrapping yarn around hook,
Then instead of inserting your hook into the stitch like a regular double crochet stitch, insert the hook around the back of the post as shown below:
Wrap yarn around hook again,
and draw it back through the post. You'll have three loops on the hook.
Next wrap yarn around hook and draw it through the first two loops on the hook,
You'll have two loops on the hook,
Wrap yarn around hook again and draw the hook through both loops,
and one loop will be left...the front post double crochet stitch!
Here's a helpful video to learn how to work the stitch too: