Crab stitch or reverse single crochet is a one of the basic crochet stitches.
At all crochet schemes crab stitch symbol would appear as:
At the scheme below you can see a stitches of crab stitch.
Crab stitch instructions
Instead of working from right to left, you work from left to right. Insert the hook in the last stitch of the previous row under the top two loops.
Pull the yarn back through the loop.
Bring the yarn over the crochet hook from back to front and draw through the two loops on the hook.
Insert hook in the next stitch of the previous row under top two loops. Pull the yarn back through the loop.
Draw the yarn through the two loops on the hook. Repeat the preceding steps in each stitch across the row.
Save this step by step tutorial as a reminder