It means that when you are away, people can see where you are. To update your position you simply (!) need a mobile phone and a GPS location!
The one that attracted my attention and made me think about using it to log my trips was the UKGSers SUYB Trip to Morocco this month. In fact as I type they are about half way through their trip.
My first "go" will be at the weekend for the SOC Kent trip across to Belgium for the Wipers Times trip to the Ypres Salient.
Should be a bit of fun!