A frost stat is a device that cuts your pump in when te temperature reaches your desired minimum, and cuts it off when it reaches your desired cut off again.
We have one fitted to our transit and it saved us so many days last year simply by leaving it on overnight, and as soon as it dropped below 4 degrees it cut in, fired up the heater, and trickled warm water round the system. Once the temp raised in the morning, it had cut off, system ready to rock, and tank full of warmish water.
Got mine from Poseidon products, you will have to google them.