This is a Waterbed Air Extractor Valve.
It is sold as a "super burper".
After filling a bed with water there will be residue air inside. This causes noisy air bubbles. This valve can be used to remove this air from the waterbed mattress.
The valve can be inserted firmly into the filling inlet of your waterbed mattress. The valve has a floating chamber inside which allows air to pass. Water will lift the floating chamber in the valve and close it. The burper just needs to be pumped up and down to remove the air - water will not pass. This valve will fit all standard waterbed mattresses.
This is a small lightweight inexpensive product. It is 7cm long. It is a handy little extra accessory to have and keep in the cupboard for when air needs to be removed. After spending a lot of time working with waterbeds I would rather use this than a conventional pump, it is cheaper, faster and easier to use. It is a better choice and even fits in your pocket. After trying this customers don't go back to conventional air extractor pumps.
This products has been sold in its tens of thousands on the world markets.