The title of Canon belonged to the religious leader of one Human society whose civilization was at the level of medieval Europe. Canons had a ring capable of summoning "divine wrath" in the form of lightning. This ring, presumed by SG-1 to be Goa'uld technology, was traditionally passed throughout history from one Canon to another.
The last Canon was encountered by SG-1 when they arrived on the planet via the Stargate. He branded SG-1 as demons due to them coming from the "Circle of Darkness" (the local's name for the Stargate), and attempted to sacrifice them, along with a girl named Mary, to a Goa'uld with a Unas (Demons) host sent by Sokar.
Eventually, after Simon, a local villager sympathetic to SG-1, fatally shot the Unas with Teal'c's staff weapon, the Goa'uld took the Canon as its next host. However, this was discovered by Major Samantha Carter, and the Goa'uld-possessing the Canon was then shot. After the symbiote died first, the Canon regained control of his body and asked for forgiveness before dying. (SG1: "Demons")