The transduction barrier was a force field that protected the Time Lords' planet of Gallifrey from invasion.
These barriers could also be raised around Time Lord space stations, and TARDISes were unable to pass through them. (AUDIO: Trial of the Valeyard)
The barrier was deactivated on the orders of the Fourth Doctor. This allowed the Sontarans to invade Gallifrey. (TV: The Invasion of Time) A squad of Daleks which infiltrated Gallifrey lowered the transduction barrier, allowing them access to Gallifrey to invade. (AUDIO: The Apocalypse Element)
In an alternate timeline, the Knights of Velyshaa breached the barrier and invaded Gallifrey. (AUDIO: The Sirens of Time)
The Eighth Doctor breached the transduction barriers to land the TARDIS outside Madame President Romana II's presidential palace. (WC: Shada)
Anti-Time was unable to pass the transduction barriers. (AUDIO: Neverland)