Flash is a multimedia publishing platform often used used to publish rich media on web pages. Flash is most commonly used on the web for animations and games, although many (including Flash's own developer Adobe) claim that its days are numbered.
Users need to have a Flash player installed onto their web browser in order to see Flash content.
Search engines cannot read any content published with Flash and therefore, Flash content alone gives no relevancy signals or value to your optimisation efforts. This is one of the reasons that most web developers, software engineers and web content creators are choosing instead to use HTML5.