A communication relay is a station that relays messages between various points, so as to facilitate communications between units.
When Colonial Heavy 798, escorted by Lee Adama, encounters two Cylon Raiders en route to Caprica, Galactica receives DRADIS and communication via a nearby relay station (TRS: "Miniseries").
After the Fall of the Twelve Colonies, Gina Inviere "helps" Kendra Shaw locate and identify a communications relay used by the Cylons. During the planning stages of this attack, Colonel Fisk notes that the relay looks like a "frakked up can opener", and Admiral Cain decides to engage this target because it is considered a light target. Upon executing their attack plan, it becomes apparent that the communications relay they attack is a staging area and that Inviere has lead them into a trap, resulting in heavy losses of both military assets and personnel ("Pegasus", "Razor").