Supposedly it gels in your stomach to make you feel full. I've not found any references to published studies to support their claims that it helps in weight loss. However, one article noted that a stomach full of brown algae could have you running to the bathroom a lot
This isn't a particularly new idea. There are a lot of diet pills that contain various fibers that absorb fluids and swell up in the stomach, to help make you feel full. They are a LOT cheaper, averaging less than $5 a bottle vs $65 per bottle of the other type.
However, most people overeat out of boredom or stress, not hunger, so taking something to feel fuller may not help many people. If you think something like this might help you, you may go to your local drugstore or VitaminShoppe and buy a bottle of cellulose fiber capsules and see if that helps.