Morse code was a symbolic representation of the alphabet, in which letters were represented by a series of sonic bursts of varying lengths. These bursts were visually represented by dots and dashes. The code allowed for messages to be compiled, one letter at a time, and sent between any two parties who could understand them.
The Tenth Doctor used Morse code when trapped in the disused Lodestar refuelling station on the planet Hurala during the 26th century. He was trapped there for five days, fourteen hours and twenty seven minutes before the Wayfarer landed there and Scrum was able to decipher the SOS message the Doctor was sending out using a spoon. (PROSE: Prisoner of the Daleks)