A flight time tracker is a tool or app that displays the latest information about the location of a plane in transit along with its estimated time of arrival. It works by relaying information from the Federal Aviation Administration and different satellites about the plane's coordinates.
Any plane that departs an official airport must file its flight plan with the FAA or similar governing body and transmit information about its location to the departing and arriving airports in order to coordinate landings and scheduling with other fights and services. Much of this information travels over radio waves that reach a traffic control tower for processing, with some also transmitting to satellites as well. A flight time tracker website or app uses this information to determine where it is on its predetermined flight path and factors in weather conditions and plane speed to determine when it is likely to arrive.
Many flight time tracker websites and smartphone apps feature interfaces that allow a user to enter a flight number to view details such as a map showing the plane's approximate location. As a plane constantly sends new data regarding its position, the tools are able to show information in close to real time, often operating at a 30-second delay.