The Light type is a fanon type that, in the earlier years, serves as an opposite type for the Dark type, and is also similar to the Fairy type, but does have a few differences.
In such earlier years, 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 literally, being based on illumination instead. Rarely, both of these concepts are thrown into a Pokémon.
In some other games, it can also be associated on freedom, as seen in Mighty and Nasty versions, and are not all good. There is even an usually evil Pokémon called Vitalight.
However, in Gen EX and Gen EX 2, it is ultimately another elemental type similar to Fire, Ice and Electric instead of goodness, and avoid the common cliche of Light and Dark being Super Effective against each others.
Many past 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.
Nowadays, the earlier concept of Light-type (the "holy" one) was criticized for being cliché and doesn't really represent the name very well, as it was often pushed to be a counterpart with Dark. Not only this was redundant does to the Fighting serving it a counterpart to Dark-type better, but it also makes the older Light too similar to the Fairy in term of either holiness or magical power.
That being said, the "holy" Light-type still exists in fanon Pokemon games time to time, or have some of its holiness incorporated in the more literal version of Light-type.
Although the modern Light-type was sometimes called as a "generic" type, it can represent the actual elemental light very well after doing efficient research and allows possibilities, such as more prominent presence of aurora, hard lights, luminescence, speed of light and lasers. This also means that this concept of Light-type is more in-line of Fire, Ice and Electric in term of elemental power overall.
Modern Light-type can also serves as a counterpart to Ghost does to Ghost-type in general represent actual darkness better than Dark-type.
There is not much changes in Mr. Yokai's Pokémon game, other than it being simply light. However, the lights must be brighter (or they must use light as means of attacking, powering up, or helping), otherwise it won't qualify for Light. This doesn't include all Pokémon which uses light; some like Lanturn are not retyped.
As time passes, Light as a fanon type was more based off goodness as a direct counterpart of the Dark-type (named "Evil" in Japan), even if some of them are more based of illumination. However, a few users like Samtendo09(t∣b∣c) prefers to make Light as an element, and not goodness, due to the fact that "good light vs evil darkness" had become too common nowadays and to avoid being too similar to Fighting and Fairy in a way, as well as allowing a proper Type that can act as a "Good-type" toward Dark-type.
That said, a mix of both (the holy/good version and the elemental version) can also exist in a few other fanon medias.
However, certain Pokémon, such as Lanturn, specifically say they use electricity; as a matter of fact, Electric would make this type feel redundant. A user named MrYokaiAndWatch902(t∣b∣c) decided to solve this by only typing dual-type Pokémon as Light if it's not produced from electricity.