Fast Travel is a concept typically found in games with large overworld maps that allows the player to instantly warp to a specific location instantly without having to take the time and effort to cross the environment. Most games that feature fast travel include a requirement that the player must discover or arrive at a location on their own before fast travel to that location is allowed.
The specific mechanics of fast traveling, however, can vary greatly from game to game. For example, in Grand Theft Auto IV, the player must select a fast-travel location on the map, and then hail a cab to travel to the desired destination. As another example, in The Legend of Zelda: A Link Between Worlds, the player calls a witch by ringing a bell, and then selects a previously accessed overworld save point where she can drop Link off.
By contrast, The Elder Scrolls IV: Oblivion allows to fast travel to a number of locations without requiring the player discover them; NPCs can inform the player of a location, and once that knowledge is shared, fast travel to that destination becomes possible. In Guild Wars, the player is often required to complete a quest or mission in order to proceed to certain areas, and certain key locations are not available for fast travel at all.