Like Rick, my guess would be a water meter if you are on city water. Some cities charge different rates for irrigation usage (possibly coupled with time of day/week/month stipulations). Call your utility and ask.
The Hunter box is definitely the timer. They are usually pretty easy to understand and you might even be able to find a manual online. Otherwise, any landscaper who does irrigation systems should be able to help you learn how to use it. If you are not a DIY person you will be calling him regularly to repair leaks, replace heads, etc.
Not sure about the climate in IA, but if your sprinkler lines in the ground are not below the frost line then remember to have the system purged every fall to prevent burst pipes. This involves a guy (usually the landscaper who services the system) coming to your house with a big honker air compressor and blowing the water out of the system.
