Answer: The Prime Meridian -- and its opposite, the International Date Line -- are lines of Longitude called meridians. There are 360 whole-numbered meridians on earth, divided into 180-degree halves, represented by the two meridians described, above. Distances east and west of those lines are measured in degrees (60), minutes (60) and seconds (60).
Because the Earth is round, the mileage between meridians can be different at different latitudes. The differences range from between zero kilometers at the poles to 111.320 Km at the Equator.
(At either pole, you can walk around the pole, and essentially walk around the earth by passing through all the lines of Longitude on earth.)
You can calculate the length of a degree based on latitude, using the reference material, below.