GMT stands for Greenwich Mean Time. Greenwich is the most popular town of London located in South East London. It is popular because of its global value of reference time zone. Meantime of Greenwich is taken as the reference point of time zone according to which times of all the time zones of the world are coordinated.
This Greenwich time was declared as global time on November 01, 1884. 24 time zone were created for different regions all across the world. These time zones are created in coordination with this global GMT time. This GMT is now named Universal Time Coordinated and abbreviated as UTC. This time zone is followed all across the United Kingdom as standard time and all the time zone across the globe coordinate their time according to this global GMT time.
Coordinated Universal Time (UTC/GMT):
If you're west of Prime Meridian, your GMT will be in front of, or less than the time at the Prime Meridian. If you're in the east the Prime Meridian, your time will be later, or more, GMT. Add the plus or minus symbol on top of the number that you have figured out in the previous step, and you'll have the time you're on GMT.