Basically, it controls your heater for you if it goes off track. You place the temperature probe into your tank, plug your heater into the unit, and plug the unit into the wall.
I put a Hydor Theo 50W heater into a 5 gallon bucket of water and set it as high as it would go. It got the water up to about 88-89 degrees. Then I left the heater set at the highest setting and plugged it into the temperature controller with the controller set at 71 degrees (it's only in C, so I set it to 22 C). The temp in the bucket has held steady at 72 degrees every time I've checked it since Wednesday morning despite the heater being set much higher. I might add, I put it in the back corner of my basement where the ambient temp gets down around 62-63 F at night.
So I feel pretty good about putting this in my 54 gallon tank! I don't think I will get one for every tank I own, but considering I have a couple hundred bucks of fish in this tank, it seemed like a good idea. I have a Hydor inline heater in this tank (which is AWESOME by the way), so I feel like testing it on the same brand somehow makes sense.
Anyway, I don't work for Azoo (or Hydor) I just found this product and believe it's a good one and thought others might be interested! I know any product can malfunction, but I feel like this creates a little bit of a fail-safe. If the heater goes haywire, the controller will handle it. And if the controller goes haywire, the heater will handle it.