Imagine that you have two points on the earth's surface, with a straight line drawn between them:
Each of these points can be described as a coordinate using some arbitrary coordinate system (for example, using latitude and longitude values), while the length of the straight line could be considered to be the distance between the two points.
Note
Of course, because the earth's surface is not flat, we aren't really dealing with straight lines at all. Rather, we are calculating geodetic or great-circle distances across the surface of the earth.
Given any two coordinates, it is possible to calculate the distance between them. Conversely, you can start with one coordinate, a desired distance and direction, and then calculate the coordinates for the other point.
The pyproj Python library allows you to perform these types of calculations for any given datum. You can also use pyproj to convert from projected coordinates back to geographic coordinates...