That fitting is installed between the hot and cold supply lines at a single location/fixture in the house, usually the farthest away from the heater. Then, a compatible tankless heater can use a pump to push hot water out while pulling the cold water back in through the cold supply line - so no extra recirculating pipe has to be installed. You trigger the recirculation with a switch at the fixture, on a schedule, or with an app/smart home device.

And you need to buy 2 if you don’t want to rebound!

“a novel ingredient composed of a linseed industry by-product, fibre and rapeseed [canola] oil”