In some parts of the world iOS SOS is a mandatory feature, and in other places, it's not something some people know is available.

The feature is a built in SOS that allows your iphone dial a local emergency number. The feature varies depending on the country you live in. The feature is built into iOS 11 in the United States. After the call is complete, iphone then sends an emergency text message unless to give the order to cancel.

As part of the text message, iphone sends your emergency contacts your exact location. It will update info to your emergency contacts if your location changes.

Further details are available here on the Apple Support page. Emergency SOS is also available on the Apple Watch in case you get one for Christmas.