The Tok'ra receiver is technology that has been used by the Tok'ra to track enemy vessels. Tok'ra operatives spent several years tagging as many Goa'uld ships as possible and eventually more than 100 Goa'uld motherships were tagged with beacons which transmitted their location via subspace to an encoded Tok'ra network. The special receiver taps into the subspace network in order to monitor the location of Goa'uld ships and to get current Tok'ra intelligence reports from operatives in the field.
The Tok'ra High Council never felt that the SGC could be trusted with the technology or the intelligence, however, during the war with the Replicators, Jacob, whose relationship with the High Council remained strained, stole one of the receivers without the Council's knowledge and shared it with the SGC, making it possible to monitor the war between the Goa'uld and the Replicators and to watch as Goa'uld ships were being destroyed or taken over.
Following the destruction of the Ancient communication terminal and stones, Carter had attempted to adapt the Tok'ra subspace communication technology to artificially emulate the Ancient technology by trying to mimic the frequencies sent and received by the stones when they were being used by Daniel and Vala. Her attempt to duplicate the technology proved effective when Vala succeeded in communicating from the Ori galaxy through the body of Daniel at the SGC.
Cross Reference: Tok'ra
Episode Reference: Reckoning, Crusade