You can fill it will water and put it inside (certain compatible) backpacks. It has a drinking "hose" that you can drink out of so that you have a steady supply of water while never opening your backpack up. They're really great for long hikes and bike rides.
You don't explain what this is and what's worse is that the mfg doesn't seem to know what it does either based on their description on their website. "Resevoir?" What is that...some kind of big water bottle?