The following is the quote from Apple:
To ask questions without pressing the Home button, plug your device into power and turn on "Hey Siri.” With iPhone 6s, iPhone 6s Plus, iPhone SE, and iPad Pro (9.7-inch) you can use this feature without plugging into power.
- Go to Settings > General > Siri.
- Turn on Allow "Hey Siri."
- In iOS 9 or later, follow the onscreen prompts to make your voice more recognizable for "Hey Siri."* This lowers the chance that other people will accidentally activate Siri on your device.
You can turn off "Hey Siri" in Settings > General > Siri. If you have an iPhone with iOS 9 or later, you can also temporarily disable "Hey Siri" by setting your device face down.
*Information about your voice isn't tracked or stored outside of your iOS device. You can remove this information from your device by turning off “Hey Siri.”