The apparent size of an object changes with distance, as we see daily. The width of your index finger may be a couple centimetres. Move it close to your eye and you can block your whole vision out of it. Move it further away and it occupies a small fraction of your field of view. All the while, the absolute width of your index finger has never changed.
Because our field of view is described by an angle, the area we can see increases with distance. As an object moves farther away, its real-world size doesn't change, but its apparent proportion of our field of view becomes smaller and smaller. This proportion of an angle is also an angle and we can describe it with milliradians.
A metre is always a metre, but we can measure its apparent size to us in milliradians, so we can tell what a metre is at any distance.
If you know how far an object is and measure the milliradians between it and another point, you know the distance between them.
Similarly, if you know the real size of an object, you can use its apparent size to calculate the distance to it. We can do this with surprising accuracy and very little effort using the mil-relation formula.
Above: 1 metre cube appearance at different ranges
