Distrans refers to a technology by which information could be implanted in animals for storage and retrieval.
The information was stored subliminally by voice. The message could be retrieved from the animal by uttering a word or phrase. The message would usually be repeated by the animal as a series of words uttered through vibrations. The sound was likened to words connected by a series of hums.
The Fremen of Arrakis, and presumably assassins used distrans animals extensively as a form of covert communication. Typically, birds were the most common form of distrans employed in the days of the old Imperium. Bats were especially useful, as they could travel under the cover of night.
Behind the Scenes
In a scene from Frank Herbert's Dune Messiah, the dwarf Bijaz, a product of the Tleilaxu, was employed as a human distrans, as was the Face Dancer Scytale. At the time it was considered to be inappropriate and robbed humans of their dignity. It also served as a plot point to demonstrate further reasons behind people's disdain for the Bene Tleilax, as they used humans in perverse ways.