The Greenwich Meridian (or Prime Meridian) is the line of 0o longitude from which all other lines of longitude are measured. The location of the meridian was agreed at an international conference that took place in Washington, D.C. in 1884.  Before that time, navigators from different countries had used a variety of 0o meridians which had led to a great deal of confusion. It is named after Greenwich in London, England which was the site of the British Royal Observatory and through which the meridian, by definition, passes.
Like all lines of longitude, the Greenwich Meridian connects the North Pole and South Pole. It passes through the Arctic Ocean, the North Sea, England, France, Spain, western Africa, the Atlantic Ocean, the Southern Ocean and Antarctica.