its a feature which is not really required at the moment, but in the future when we are all flying GPS approaches the predicitive GPS function looks at your ETA at arrival (or at a given waypoint) and calculates if you will have sufficient GPS coverage. It shows this either as a Y or an N for yes or no for various times before and after your ETA. It will prob all be Y's when you look at it but just play about with it and deselect some GPS's (this wont actually deselect GPS's but just carries out the predicitve GPS function as if that particular GPS satelite signal had failed). I think you enter the number of the satelite (1-24???) to deselect and you will start to see some N's appearing. ie. you wouldnt have sufficient GPS coverage to fly a GPS approach at that time if those satelites failed.
Good luck.