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It is a aux box that allows you to raise the rpms of the engine for a pto or you can push the charge button which will maintain battery voltage under different loads like while winching or using an inverter.
I got one in my 96 and love it, especially in the winter, start er up in the morning, hit the E-brake and before I get back in the house she's purring along at 1250 making heat and electricity!! You can find em on ebay for around $70, real easy to install, most trucks already have a wire source with the right plug in on it already, either behind the ebrake pedal or mine was right above the obd2 port. That pic you posted is also the newer model with the digital screen, the older ones like mine only have the buttons and the red lights.... no biggy as my dash tach tells me what the rpms are and my super tuner 3 radio tells me the voltage!!
it is a high idle/voltage controller, they were used frequently on emergency vehicles, that way when you pull up on scene you set it and the truck idles up so all the lights don't drain your batteries while it is sitting there. it is a nice little convenience to have when it is cold out as well