In the context of YouTube, Flash refers to Adobe Flash Player. YouTube uses the Adobe Flash Player to display the videos you watch on the site every day. Flash Player is a plug-in for your web browser, and like most software it is updated periodically by Adobe.
We have found that users with the most recent version of Flash, Flash Player 10, experience improved performance such as smoother playback when watching videos on YouTube. We encourage you to upgrade to the most recent version of the Flash Player by visiting the Adobe download site.