The prefix of a phone number, in the United States, is the first three numbers of a seven-digit phone number. For example, in the number "555-1212," the prefix is "555." The prefix is a holdover from when telephone systems used manual and mechanical switching.
In the early days of the telephone industry, telephones within an area code would be controlled by a switch; this was initially a switchboard controlled by a human operator. Each switch was designated by a name and eventually a number and controlled up to 9,999 phone lines, each represented by a four-digit number. The human operators were eventually replaced by electromechanical switches, which were, in turn, replaced by computerized switching.