You're living in a cave without wifi if you haven't at least heard of the "Internet of Things." Everyone says it's the way of the future, and people are either really excited about it or completely terrified (sometimes maybe both at the same time).
We've got all of these great devices that do all of these amazing things, but so far none of them do a very good job of talking to each other. Apple is hoping to change that with HomeKit, which is their platform to give you more control over the things you interact with on a daily basis and let them communicate in whatever ways you specify.
So what can it do? Imagine that you leave work and start heading home. Siri already knows you tend to head straight home from work, and your thermostat knows that it takes 18 minutes to get your house up to the temperature you like it, so when you're 18 minutes from home the thermostat starts warming up the house. As you pull up to the house your garage door opens for you, and since it's after sunset the lights in the hallway come on automatically (but not too bright, since you like it dimmer in the evenings).
This is the kind of stuff HomeKit is capable of—and not at some point in the future, but right now. Of course in order to take advantage of it, you have to have devices that support HomeKit, and there aren't a huge number of them yet. That's because Apple set the bar pretty high for security on the devices so that someone else can't hack into them and find out when you're not at home, for example. Security is one area where Apple has traditionally excelled, but this comes with some hurdles for manufacturers, and as a result development of HomeKit devices has been slower than with other platforms.
If you're thinking you might like to experiment with it a bit, we would recommend you take a look at the Lutron Caseta system. It's easy to setup, the entry cost is fairly low, and it's very reliable. You can read more about it at Lutron's page.