'Flash' was introduced in the Time Spiral block as a formalization for a previous rules text stating that a creature may be cast as an instant.

702.8. Flash

• 702.8a. Flash is a static ability that functions in any zone from which you could play the card it's on. "Flash" means "You may play this card any time you could cast an instant."
• 702.8b. Multiple instances of flash on the same object are redundant.

Translation:

• 702.8a. Flash is always active and puts in place the ability for this creature to be cast any time you can cast an instant.
• 702.8b. Flash is redundant, multiple instances does no good.