It's a 24 hour hand basically, but you can set the time zone that it is showing. Pull the stem in the datesetting position and wind it backwards. The GMT hand will move (or jump in one hour incriments depending on your movement) so you can position it where you want. You read the GMT hand from the bezel on your watch, not the dial.
Say for example you are in UK, it's 12pm, but you have a call to make at 7pm in China which is 5 hours ahead. You set the GMT hand to show 17:00 hours.
If you're using a GMT you can also set a third timezone against your regular hour hand by rotating the bezel. So a GMT can track 3 time zones, an Explorer II can track 2.