In espionage1 parlance, a cut-out is a mutually trusted intermediary, method or channel of communication that facilitates the exchange of information between agents2. Cutouts usually know only the source and destination of the information to be transmitted, not the identities of any other persons involved in the espionage process (need to know3 basis). Thus, a captured cutout cannot be used to identify members of an espionage cell4.
Espionage on Wikipedia
Agents in espionage
Need to know on Wikipedia
Clandestine cell system on Wikipedia