I’m using US terms.

what you can do with it:

The popcorn stitch is a one-sided stitch, so you don’t want to use it on something where both sides are showing.  Here I’ve made my popcorn stitches stacked on top of each other, but you could stagger them, too.

how to do it:

If you want to do your popcorn stitches alternated with double crochets, like I have here, then you’ll want to chain any odd number +3.  In the fourth chain from the hook, make 4 double crochet stitches:

Next, take your hook out of your work, making sure not to pull your working yarn.  Insert your hook under the top two loops of the first of the four stitches, then put it back through your working yarn:

Pull the working yarn through:

You’ve made a popcorn stitch!  My finished product here is popcorn stitches with dcs in between them, and the second row was just sc: