As the video explains, the vehicle goes by the acronym LCAC (pronounced like "el-cack"), which stands for "landing craft air cushion." The acronym sums up its function well also. Like all hovercrafts, it moves on a cushion of air, and this one is used for moving military equipment and personnel from ships to the shore.
Now something that should be mentioned is that not all 20,000 hp goes to moving the LCAC forward. According to the video, there are four engines on board, and two of them are there to generate the air cushion. The other two are used for forward movement. So technically only 10,000 horsepower is being used for making forward progress. But that's still 10,000 horsepower, and it's enough to get the massive machine moving up to 80 mph.
There are more great details about the LCAC worth checking out in the video above. You'll even get to see Jay swap his denim for a khaki jumpsuit and take the reigns for a little bit. It's a good thing there isn't much to hit in the ocean.