In a world I am building, there are two sapient species, named Ae and Be. Ae are tall but blind, while the Be are short and sighted. While it has been asked how a blind species would discover another species' ability to see, on the other end of the spectrum, how would a species with sight explain what sight is?
How technologically advanced are the two species? If they've reached the appropriate tech level, then a sighted species should be able to explain that they have organs that detect electromagnetic radiation in certain wavelengths.
I'd assume that the blind species has some equivalent to sight; echolocation, for example. The blind species likely builds up a picture of the world much the same as the sighted do. It should therefore be as simple as saying "The same way you can hear textures of different surfaces, we see different colours." Looking for common experiences and using them as analogies is a very effective tool.
EDITED TO ADD:
Surprisingly enough, we may find that the unsighted species understands the world in remarkably similar ways to the sighted.
Some humans have developed echolocation. People who were born blind or who lost their sight in infancy have demonstrated the ability to rewire their brains such that data is passed from the ear to the visual cortex of the brain, and essentially processed as an image. They can 'see' well enough to ride a bicycle in traffic, or distinguish a metal fence from a wooden one. They can perceive and appreciate visible beauty, and describe the picture they get as "very rich".
The evidence suggests that bats perceive the world the same way. Sure, they don't see colour the way we do, but they do seem to perceive texture much more precisely than we can with our eyes. That's why I suggested that we could describe sight to the unsighted using the analogy of texture for colour.
They can simply say that they can detect the shape of object from a distance up to several kilometer, which is the basic definition of sight. Next, list the restriction for correct sight (light level required, impossibility to see through thing, intact eye, etc...).
With this, the blind specie will be able to understand what sight can do, even if more complex things like colour will be more difficult to understand.
Explaining how sight works would be poor and clumsy at best, although it would give the blind race a starting point to trying to understand it.
How do we understand Echo Location? We put bats in a dark room and gave some of them earplugs.
Admittedly, Tech-level would affect how long it took to understand. For example, to understand echo location, you first need an instrument that can pick up sounds you can't. Over time the blind race would come to understand light as 'some form of radiation' that sighted races can detect with their eyeballs. Since they don't know what color is, they would probably learn to associate colors with sounds, and might 'visualize' a picture as a sort of weird melody. (similar to our distorted view on what echo location 'looks like')
The classic answer in our world is to prove sight is to hand a blind person a camera. The blind person goes into a private room, takes a picture of her hand holding up some number of fingers, takes camera back, and sighted person states how many fingers.
In your case, the short sighted people might try to pass off sight as magic.
Maybe you could relate it to heat radiation. Just as skin can sense heat from a distance without any sound, the organs called eyes can sense a different type of radiation called light. And just like sound can bounce off of objects to determine different aspects of that object, light bounces off everything at once to reveal many other different aspects of objects.