The prime meridian is the planet’s line of zero degrees longitude. Slicing the earth along the circle of the prime meridian would divide it into the Eastern and Western hemispheres — but it’s probably better to leave it in tact.
Sometimes called the Greenwich Meridian or the International Meridian, Earth’s prime meridian crosses the original site of the Royal Observatory in Greenwich, England. It is the circular line by which other longitudinal lines, or meridians, are referenced — otherwise it wouldn’t be prime. You can still have a prime meridian without Greenwich, England, though; you just need to be on the moon, Jupiter, or some other celestial body.