The Prime Meridian is the meridian (line of longitude) that goes through the Royal Observatory, Greenwich in London it is also known as the International Meridian or Greenwich Meridian. Other longitudes are given as east or west of the Prime Meridian.
Lines like this are not actually lines on the ground, but are ways of saying where a place is on the globe. The ones that run horizontal (east to west) are called latitude. They tell us how far a location is from the equator. The other lines are vertical and are known as longitudes.
Such is the Meridian's fame that each year, hundreds of thousands of visitors from all around the world make their way to the Observatory to stand astride the Line. But its position is marked in hundreds of other places too. On its path from pole to pole, the Meridian passes through England, France, Spain, Algeria, Mali, Burkina Faso, Togo, Ghana and Antarctica.