Adobe Flash is a software platform frequently used on the web for providing animations, interactive applications, advertisements, or streaming audio / video. Flash is a proprietary technology that requires additional plug-ins to view in a web browser.
While some more sophisticated search engines like Google have had some success in indexing content found in Flash, most search engines have great difficulty or do not attempt to crawling Flash content. Links in Flash may not be followed by search engines, so using Flash in site navigation can have a hugely negative effect on your site's indexation.
While a powerful technology, usage of Flash on the Internet has declined sharply in recent years, due to the rise of HTML 5, which can perform many of the same functions as Flash, but is an open standard that offers better compatibility, is easier for search engines to crawl, and uses less processing power.
It is a best practice for SEO to minimize the use of Flash on websites, especially for content containing keywords, navigation, or site-wide. Instead, try to use HTML 5 techniques to create a similar experience on your site.