Progressive enhancement is an approach where core content and functionality must be available to all users through basic HTML and CSS, while enhanced features like Flash, JavaScript, and richer formatting can be optionally loaded to provide richer experiences to browsers that support them. This helps sites work for all users, including search engines that may not be able to see Flash or execute scripts. Tools like NoScript can help test how sites look without scripts. Methods like SWFObject allow replacing HTML content with Flash movies while keeping the same HTML content available for search engines and non-Flash users. It is important to use these techniques properly and not rely solely on Flash to avoid search engine and accessibility issues.