The Internet of Things or IoT for short is the means by which we can control devices over the internet. It can also be how one machine talks to another sometimes called Machine to Machine or M2M for short.
Smart houses often use this to control the heating for example by using your phone to switch the heating on or control the temperature. This is just one application of the technology. There is a vast array of uses.
If you want to learn about this technology one of the best places to start is by using the particle photon it is a small device that connects the wifi in your home with a website from which you can read data or control something. I will be providing an introductory guide in the near future.