The Prime Meridian is an imaginary line drawn from the top to the bottom of the globe earth map, or, in other words, the Prime Meridian is an imaginary line which runs from the North Pole to the South Pole of the earth and passes through Greenwich.
The Prime Meridian is said to be imaginary because no such line exists on the earth itself, but we draw it on our globe maps and other maps to help us pinpoint locations.
The Prime Meridian is starting point of all other lines of longitude. It is said to be 0 degrees. There are 180 lines going west of the prime meridian and 180 lines going east of it. Each line represents one degree.
The Prime Meridian divides the earth into the Eastern and Western Hemisphere, while the equator, another imaginary line, divides the earth into the northern and southern hemisphere.