A meridian is an imaginary line of longitude drawn along the surface of the earth from the North Pole to the South Pole.
Geographers today measure these lines from what they call the Prime Meridian. It is the line of longitude that goes through the Royal Greenwich Observatory in Greenwich, England. They decided this at the 1884 International Meridian Conference.
Meridians, as lines of longitude, are measured in degrees. The Prime Meridian, as the starting point, is 0 (zero) degrees. Lines west of the Prime Meridian are either called west, as in "the longitude of Los Angeles, California is 118 degrees west", or as a negative number: "the longitude of Los Angeles, California is -118 degrees". Going the other direction, east of the Prime Meridian is always said as a positive number: "The longitude of Mecca is about 40 degrees east," or just "The longitude of Mecca is about 40 degrees."