Proximity turns a distance into a score from 0 to 100: 100 is the secret country itself, and 0 is the opposite side of the world.
It is calculated from the distance and half of the Earth’s circumference, so it is just another way to read the kilometres.
The coloured squares in your shared result use the same scale.
See also What the arrows mean. For tips, read the Closer or Farther tips and strategy.