Klaxus: The short answer is yes.
The ring grants the Melee +1 and Ranged +1 traits for all of the Priestesses attacks which deal light damage.
Essentially, that adds 1 die to each of her attacks, which could be a weapon that deals light damage (like her Staff of Asyra), or an attack spell like Pillar of Light.
Note that the Melee +1 and Ranged +1 traits have certain restrictions. For example, Ranged +1 does nothing for a zone attack. So if the Priestess casts Blinding Flash, the attack does not gain an extra die for each object in the zone.