What does prime meridian mean

 In order to describe how far east or west a place is, you have to know
where the measurements begins ... so you can say how far it is east
or west OF WHAT.

So around the time when it started to become common to take long
ocean voyages  far east or west of your home port, the major countries
in the world got together and agreed on where all east or west measurements
should start from ...  the place that everyone would call "zero".

THAT's the Prime Meridian.  It's an imaginary line on the Earth's surface
that joins the north and south pole.  That much doesn't tell you where it is
yet.  It could join the north and south pole and pass through Chicago, or
Jerusalem, or the kitchen in your house.  The thing that everyone had to
agree on was where the 'zero' line should pass through.  And the place
they agreed on was:  A groove in a metal plate in the floor of the King's
Royal Observatory in Greenwich, England.  (Pronounced GREN-itch.) 
To this day, that's the line that everybody begins their east or west
measurements from.  The angle on the Earth's surface from that line to
any location is called the east or west 'longitude' of that place.

On its way from the north pole to the south pole, the Prime Meridian
crosses parts of England, France, Spain, Algeria, Mali, Burkina Faso,
Togo, Ghana, and Antarctica.

Chicago is about 42 degrees West of the Prime Meridian, and New York
is about 74 degrees West.  Beijing is about 116 degrees East of it. 

And if you and your best friend stand on the Prime Meridian, and you go
180 degrees East and he goes 180 degrees West, you both meet at the
same place, coming from different directions, because 180 degrees is
half-way around the Earth.