This is a relatively new service, which you may have seen advertised recently which can help in locating you if required, but does require your mobile phone.
Firstly it should got without saying your phone is not a substitute for a map and compass enjoy the hills but please be Adventure Smart
In simple terms the idea is to split the world up into 3m squares and give each square a 3 word reference (as the name would suggest), the idea being this is easier and more readily communicated and cross checked by the services.
What3Words site is an independent site.