In a nutshell, you're working single crochet in the opposite direction from whatever is correct for you. For right-handed crocheters, like me, we normally work single crochet in a row from right to left - after turning the work, of course. When we work the crab stitch, we will work from left to right without turning our work.
For left-handed crocheters, you normally work a row left to right and reverse single crochet is then worked right to left without turning.
The yarn does this lovely wrap around itself and through each stitch, which creates a thick, bumpy edge.
I've turned simple swatches into face scrubbies and crocheted circles into coasters with this stitch. I've even used this stitch to join my work and create a "country patchwork" edge for this Christmas stocking.
See how it makes a swirly, spiral-like edge? I just LOVE it! This can edge small projects, like a granny square, or large projects like an afghan. It's so lovely.
Check out this video tutorial that shows you how to work reverse single crochet in just a couple of minutes:
