Flash, (not flashback) was introduced as a keyword when TIme Spiral was released. Like Lifelink, Defender and a few other keywords, the mechanic had existed for a long time before it was given an official keyword.

Flash appears on permanent cards and changes when you can cast them. Normally you can only cast permanent spells during the main phase of your turn when the stack is empty. Flash allows you to play these cards anytime that you could play an instant. The only real restriction is that you must be able to play an instant then which means that you have priority.