Flash is a keyword ability that lets you cast a card at instant speed. The reminder text spells it out: "You may cast this card any time you could cast an instant." Normally, you can only cast creatures, enchantments, artifacts, and sorceries during your own main phase, when the stack is empty. With flash, you can cast the card whenever you have priority—on your opponent's turn, during combat, or in response to another spell. That makes it a tool for surprise.
A flashed creature can ambush a blocker or block an attacker unexpectedly. A flashed enchantment or artifact can provide a timely answer or combo piece. Holding up mana for a flash card also keeps opponents guessing, since you can either cast it or use that mana for something else. The oracle examples show it can also be granted to other spells ("as though they had flash") or combined with other keywords like convoke, making it a core timing tool.
