The Prime Meridian (0°)

This is alot like your last question. But here is my answer agian.

A meridian is one of the 24 (well 25 if you count the international date line) artificially set lines going from the north pole mostly straight south to the south pole about every 15 degrees of longitude on the globe that set up the ~24 time zones(there are a few places that are not following the proper one).

The Greenwich (only 1 h as far as I know) is the most important of them as it is the origin line, that is it roughly follows 0 degrees longitude and all others time zones are based on it(IE add or subtract 1 hour per time zone you are away from it). It is call that because it passes through Greenwich England where the gentlemen who create the system of dividing the world that way were meeting.