Chief Archon was the leader of the Asgard High Council at the time Colonel Jack O'Neill attempted to solicit the Asgard's help with the K'tau's sun. He informed O'Neill that the Asgard could not intervene, as it would be a violation of their treaty with the Goa'uld. (SG1: "Red Sky", "Fail Safe")
Behind the scenes
The Asgard Council leader has never been named, and is usually identified as Chief Archon in transcripts.