Apple Watch has a function that allows you to quickly call 911 or the appropriate local emergency services. And after your call is finished all of your designated emergency contacts in your iPhone Health app are alerted by text. A user on Reddit recently told a story that shows just how useful this was when he was in a car accident and didn’t know where his phone landed.
Press and hold the side button (the broad button, not the crown) for a few seconds. You’ll be prompted with an option for “Emergency SOS”. Just slide the SOS slider to the right. If you accidentally invoke this you can firmly press the screen to cancel.
Your iPhone needs to be within Bluetooth range (usually about 20 feet), or your Watch needs to have Wi-Fi calling enabled with access to a known Wi-Fi network.
For full details see Apple’s document on the subject.