Flash is a blinksummoner spell that activates instantly upon being cast, teleporting the player's champion a short distance in the direction of their cursor.
Flash can easily be used to escape from or catch up with a foe. It can be used to dodge or land abilities. It can also be used to move over walls, either for a quick escape or in response to an opponent's dash.
Flash cannot be activated if the player is affected by hard crowd control.
Flash and Teleport cannot be cast while silenced. This does not hold true for other summoner spells.
For a time, the inclusion of Flash in the game was hotly debated and Riot Games Inc. even considered removing it from the game entirely, with parts of the community for and against removal. As a result, it has suffered several nerfs.
One of the pieces of trivia in the loading screen states that it takes 3 hours and 20 minutes (40 Flash uses) to travel from one fountain to another only with Flash and without masteries.