Its a feature where you can set up your phone to do certain things based on 'trigger' actions/times/locations. They have a lot of 'samples' to look through, which you can customize.
For example, you can set the phone up to go silent at night while you sleep, or you can set it up to open a certain app when you plug headphones in, or you can have screen brightness dim and ringer volume go up to 100% when you attach to your home wifi. Those are just a few examples. The possibilities are basically endless.