What is the H on the pump??

I was in the process of hooking my new fass up 150/240 ans I have to t for tank. hooked up.. the return hooked up and the supply hooked up put on the end where the return is an H with a red shiiping plug in it whats it for??

Heater

If you wanted to run coolant or something similar through the pump in really cold weather you could.