The item is relatively rare and oftentimes only a handful exist in an entire game. More often than not they can only be found by visiting Villages, by recruiting a new character that is holding one, by steal from enemy with a Thief, or by finding one in a chest. In Radiant Dawn however, they can be purchased in Aimee's bargains on an extremely rare occasion.
The Light Rune has only a single use. It can be used to block a certain space on a battle map for an amount of turns in battle by placing it. Possibly, this may be because when activated it produces large amounts of light and heat that would harm any who tried to pass through it. This makes the tile the Light Rune is placed on impassable to all characters, friendly, foe, or otherwise. It is due to this effect that it is commonly used to create bottlenecks which the player can tactically use to pick off opponents.
The exact properties of the Light Rune's magic are sketchy at best, although it is suspected to be related to Light Magic due to its nature. The Light Rune, along with the Mine, is one of the only items that can be placed on a map.
Most of the Sages on Chapter 3-11 of Radiant Dawn hold Light Runes. This coupled with the fact there are Pits all throughout the stage that can paralyze a unit for a turn can make it a very hard level to beat as the enemy creates bottlenecks.