It is the term used for Greenwich Mean Time. The Zero degrees longitude parallel runs through Greenwich in London, U.K. It runs from the north to the south pole. Half way around the world, also running from pole to pole is the 180. degree longitudal line. Every other line of longitude is either east or west of these two lines. If east, it could be Monday, whilst off to the west it could still be Sunday.
A new calendar day starts at 180 degrees. Hence, New Zealand heralds in the New Year three hours before Australia and is ten hours ahead of London, which is about three hours in front of New York, which is about three hours in front of Los Angeles, et cetera
Now it gets a little trickier. Let us say it is midnight in Greenwich London, the start of a new calendar day. If we move to the east the time is ealier if we move west the time is later (it's still yesterday) This is because the earth is revolving towards the east.
All these local time zones would create almost insurmountable problems when it comes to navigating ships and aeroplanes across these time lines (which are all mental concepts made up by human beings, by the way) So it was decided many years ago that the world would use Greenwhich Mean time. Greenwich mean time is Zulu time - comprehendi?