The idea behind Perimeter is simple: your phone knows where you are. Your phone knows what you want to happen (because you told it). So your phone should make that happen for you.
Perimeter is a home automation system that uses user location to control home appliances. For example, when you leave your office, your furnace should turn on and heat up your house. When you turn onto your street, your lights should turn. And when you pull into your driveway, the garage door should open.
Perimeter can make this happen.