An SOS was an ancient Earth term for a type of distress call. Simply Morse code for "S·O·S", this signal was displayed with the sound of three dots followed by three dashes followed by three more dots (··· −−− ···). (ENT: "Vanishing Point")
Picard had this chip on him when he transported aboard the Artifact shortly afterward, but lost the chip there. Elnor would later find and activate the SOS while stranded on the Artifact after covering Picard's escape. (PIC: "The Impossible Box", "Nepenthe")
This call dates back to the early 20th century, when broadcast signaling was still a new science. An international convention met to debate and select a "universal" distress call, and "SOS" was selected by most. It became a universal call for assistance in 1912, after the sinking of the RMS Titanic.