Morse code was a widely-used method of encoding messages. In Morse code, text was represented by a series of dots and dashes, which in turn could be communicated via a flashlight (a dot meant a short blink, a dash meant a long one). This was often used to relay distress signals in the absence of telecommunications.
During the White House siege of Day 7, Agent Aaron Pierce was wounded while escorting Olivia Taylor out of the building. Unable to go on, Pierce instructed Taylor to go to the window and flash Morse code for "Code Green," which would signal the Hostage Rescue Team to move in. ("Day 7: 7:00pm-8:00pm")