CQ is a "general call" signal sent by wireless radio operators using Morse code. A CQ was transmitted automatically by the SS Botany Bay and the code was received by the USS Enterprise. Although Lieutenant Uhura interpreted the call, Captain Kirk indicated that he understood it, suggesting that Morse code was still in use in some fashion in the 23rd century. (TOS: "Space Seed")
Sources suggest that early drafts of the script for "Space Seed" had the Botany Bay transmitting an SOS signal, but NBC had concerns about broadcasting the international distress signal. (The Star Trek Compendium)