As the promotional material explains it, the bag compresses your items so you can take a carry-on instead of a suitcase, saving you extra baggage fees.
You simply put the folded clothes in the bag to the "stop" line, run the slider twice across the zipper to seal, then roll the bag from top to bottom and squeeze the air through the one-way valve. The vacuum seal reduces the bag size.
When I tried it, the shrinkage was not as much as shown in the commercial, but I could see a difference. Space Bag To Go clearly did not reduce the filled space by half, but there was definitely room to pack a few more items.
On a scale of one to four, I give it three and a half stars, with two caveats: one is wrinkles. The vacuum seal compresses your clothes, so if they're at all prone to wrinkling you'd better have an iron handy. The other caveat is weight. These bags do give you more space for packing, but if you pack in more, you're adding weight to your luggage.
If you're not careful, that can mean extra fees for exceeding the weight limit, especially with your checked baggage.