The latest version of the Google Chrome web browser (version 55) has finally kicked Adobe Flash to the curb, by giving preference to HTML5 on most websites. There are a small number of exceptions to this, at least for a time, with YouTube and Facebook being notable ones.
Flash and HTML5 are different methods websites can use to allow you to view multimedia content - audio, video, animations - and Adobe Flash has been subject to a lot of security vulnerabilities over the years, hence the push to move to HTML5.
The Chrome browser should update itself and in day to day use, you really won't notice much difference - although if a site only supports Flash, you will likely get a prompt before Chrome will play that content, which is another nod to security.
With Chrome being the most popular browser out there, the choice to eschew Flash should help with the adoption rate of HTML5 going forward.