Temp control keeps your coil from rising above a set temperature that you set the mod to. It gives you an even temp throughout the time you are holding the fire button. It's a more consistent vape. Also designed to prevent dry hits when running low on liquid.
You have to use the nickel occ heads in your sub tank, they are designed specifically for the temp control option. They may or may not come with the tank, mine did not. If you don't have those coils, just use normal temp mode until you get some.