It's a timer shutoff that's programmable to remain on for set times after the ignition is turned off. In theory it should time out long before the connected accessories drain the battery enough to prevent it from re-starting.
I use them and I really like them. But unless you have radios that you commonly leave on or you have to leave warning lights running without your engine running then they might not be for you. I use them to control the power to my 2 way radios so that they come on automatically when I start my explorer and they stay on for 1 hour after it is turned off. That way they aren't on and off all day when I travel but when I go in for the day they turn off and don't run my battery down. You can set the timer to various lengths of time or it can simply monitor voltage to choose when to turn the load off and prevent a dead battery. I more or less just like not having to remember to turn my radios off because I have been known to forget and come out to a dead battery if I don't drive everyday.
Basically what Radio611 said, you can use it any way you see fit, if your vehicle sits on for weeks on end I highly recommend it, it can use any combination of the timer and voltage monitor you choose and can run relays/solenoids if needed for larger loads. If I didn't have my equipment all on the Retained Acc Power in my Silverado, I probably would have went the charge guard way to be sure my equipment turned off when needed, everytime you kill a battery that's 10% of its capacity gone. IMO well worth the money.