It tells you how far east or west you are from an imaginary line that runs from the north pole to the south pole and that passes through Greenwich, near London, England. You get up to 180 degrees...
They're both angles used to locate points on the Earth's surface.Other than that, they don't relate at all, in the sense that a changeof one coordinate doesn't change the other one.
On the equator, it's roughly 69.1 miles (111 km). At the poles, it's zero.Anywhere in general, it's 69.1 miles (111 km) multiplied by the cosine of the latitude.