Fanon Light type Pokémon appear to be prominently some sort of "holy" type, somewhat like the Fairy type but more serious and less cute. A good bunch of them are largely based on the concept of hope or good will. But sometimes, the Light type is taken too literally, being based on illumination instead. Rarely, both of these concepts are thrown into a Pokémon.
In some other games, it is also based on laws and freedom, as seen in Mighty and Nasty versions, and are not all good. There is even an usually evil Pokémon called Vitalight.
Many Light Pokémon based on animals are also good-hearted, a symbol of hope or something similar to either concept. Animals not based on hope or goodness are usually the ones that prominently have or use illumination, for example to lure in prey like anglerfish.