Not absolutely sure, but it looks like a PRV (pressure reduction valve). Somewhere in the plumbing, there should also be an expansion tank. While the thing should not leak, if the expansion tank (bladder) has failed, the pressure can rise and the weakest link will leak. While a PRV can be rebuilt, the rebuild kit is often about the cost of a new one, and that is usually the better way to go. The bolt on the end is adjusted to change the pressure setting, then the nut is tightened down to hold it there.
You may want to pick up a water pressure gauge, screw it onto say the drain valve of the WH, or any other hose fitting in the house, and check the pressure.