In 2183, human scientists uncovered a Prothean technological artifact on the planet of Eden Prime. While repelling an attack by Saren Arterius' geth allies, Commander Shepard's squad came into the artifact's proximity and set it off. The object, described as a beacon, sent the Commander jumbled sounds and images intended to warn of the imminent Reaper invasion. The beacon overloaded during the process and violently exploded.
While the vision it provided was dismissed by the Citadel Council as a highly subjective experience, since that time, the Council's Committee on Paleotechnology have analyzed the fragments of the beacon in an effort to understand how it communicated. What they found was not quite the "telepathic technology" that the Alliance marines described in their reports.
The beacon was equipped with a mass effect field generator to hold its subject immobile, a necessary step to minimize collateral damage during the rest of the process. The beacon then nearly instantaneously scanned the structure of its target's optic and auditory nerves as well as many parts of the brain responsible for memory. Once its expert system determined the proportions of these key features, the beacon stimulated them with pinpoint electromagnetic and other irradiative energy, using powerful programs to recreate any images or sound the beacon required. In principle, this was similar to the popular entertainment device known as a "simstim," but vastly more comprehensive--simstims direct the senses, not memory.
The Normandy's medical officer noted that the beacon delivered more information over the space of seconds than a simstim could do in hours, and managed to do so to a species whose physiology was completely unknown to its designers. To a paleotech, the fact that Commander Shepard needed assistance to decipher the message is a minor footnote compared to the Protheans' accomplishment-getting any of the message to Shepard at all.