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.
For a time, the inclusion of Flash in the game was hotly debated and Riot 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) to travel from one fountain to another only with Flash and without masteries.
Theoretically, it would take 2 hours and 24 minutes to travel from one fountain to another only by using Flash, and with 3 points in Summoner's Insight, and the Distortion enchantment.