The yellow line that travels from north to south on this image is the Prime Meridian, also known as the Greenwich Meridian. This meridian goes from the north pole to the south pole, and passes through the place called Greenwich, England, by which it got its name.
The meridians are essentially lines that go from north to south, connecting the two poles, and also connecting each other in spot on both poles. They are imaginary lines, and they serve for calculating the longitude.
