"The enemy has engaged counter-measures that are preventing our ability to transport."

—Hermiod

Jamming codes are a counter measure to Asgard transporters used by the Wraith. Developed quickly after the arrival of the Daedalus in the Pegasus galaxy, the jamming codes prevent a transporter lock, which effectively neutralizes the transporters as a weapons delivery system into Hive Ships. Even with the advent of the Wraith civil war, the codes are still used unchanged.

During their brief alliance with the Atlantis expedition, the hive of Michael Kenmore sent their jamming codes to the Tau'ri, but they proved ineffective against when the hive ship broke the alliance.

It is likely that the jamming codes were originally developed when the Wraith encountered and fought the rogue Asgard faction after winning their war with the Lanteans; this would explain their quick appearance and effective use on an, at the known time, alien technology. (ATL: "The Siege, Part 3", "The Hive", "Allies", "The Lost Tribe")

When in close proximity to a Black hole, the jamming codes are ineffective. (SG1: "The Pegasus Project")