The term 'telegraphic signal' refers to a type of communication that uses electrical signals to transmit information over long distances. It is composed of two parts: dots (represented by the letter 'D') and dashes (represented by the letter 'T'). The dots and dashes are used to represent letters, numbers, and symbols, which can then be decoded by the receiver. Telegraphic signals were used extensively in the 19th century for communication between distant locations, and are still used today in some forms of communication.