D-STAR (Digital Smart Technologies for Amateur Radio) is a digital voice and data protocol specification for amateur radio. The system was developed in the late 1990s by the Japan Amateur Radio League. There are other digital modes that have been adapted for use by amateurs, but D-STAR was the first that was designed specifically for amateur radio.
There is a good explanation of D-Star and how to use it on the DStar101 website.
In order to use DSTAR on GB3RF you will need to set your radio with the following settings: