A mixing valve is a device used to avoid scalding yourself on a pipe attached to a boiler. It functions by mixing hot water with cold water so the external pipes are a safe temperature.
Automatic mixing valves come with a temperature sensing mechanism that detects when it is necessary to add cold water to the mix. They can be adjusted so the target temperature is as desired by the user.
Unlike automatic mixing valves, manual mixing valves do not come with a temperature sensing mechanism. Instead they come with gate valves that must be manually adjusted by the user. Typically, the valve is adjusted a few times a year as the seasons change. These are mostly found on older boilers.
On manual valves, twisting the mixing valve clockwise admits less cold water to the system, making the pipe hotter. Twisting the valve counterclockwise makes the temperature of the pipe colder.