What is a tempering valve

To temper hot water you open up the cold water side until the desired temperature is reached.
A tempering valve is the valve that feeds cold water to your bath when it's too hot. Regards, Tom

There are several kinds of tempering valves for a number of different applications:

A tempering valve can be installed on a indirect fired HW storage tank (in combination domestic/hydronic heating systems) to reduce the temperature of the stored domestic water -- This is accomplished by mixing cold domestic water with overly hot water to bring the water temperature down to more manageable temperatures for domestic use.

A point of use tempering valve can also be installed on a lavatory or hand sink as an anti-scald device -- Usually in commercial applications where the domestic water must be at a higher temperature for dish washing and glass washing machines.

And some of the more stringent Plumbing Codes require tempering valves when non-pressure balanced tub or shower valves are installed or when two handled valve sets are used in deck mounted tub installations.