What is Greenwich mean time?

Greenwich Mean Time (GMT) is the global time standard. The prime meridian at Greenwich, UK, is the reference line for GMT.

it is the time used to set all clocks all over the world. places like London use GMT as their time. Every country East of the UK sets its clock behind GMT and everywhere West sets its clock ahead. eg:

Alaska's capital, Juneau, is East of the UK, so Alaska has the time zone GMT-11hrs. France's capital, Paris, is West of the UK, so Paris has the time zone GMT+1hrs. So when it is 12noon in the UK it's 1am in Alaska and 1pm in France.

What number comes after GMT is determined by lines which are set at even intervals around the world. However, in many places the lines are bent to fit around country borders, state boundaries and islands. here is a good site which shows clearly all of the time zones of the world:
Greenwich Mean Tim is a global time standard. This was a term used originally as a way to describe the mean solar time that was used by the Royal Observatory located in Greenwich London. This standard of telling time is used in areas including South Africa, Australia, Malaysia, Pakistan, and other such countries throughout the Eastern Hemisphere.