This rules triggers when PageSpeed Insights detects that the size of one of your resources could be reduced through minification.
Overview
Minification refers to the process of removing unnecessary or redundant data without affecting how the resource is processed by the browser - e.g. code minify text comments and formatting, removing unused code, using shorter variable and function names, and so on.
See preprocessing & context-specific optimizations to learn more.
Recommendations
You should minify your HTML, CSS, and JavaScript resources:
To minify HTML, try HTMLMinifier
To minify CSS, try CSSNano and csso.
To minify JavaScript, try UglifyJS. The Closure Compiler is also very effective. You can create a build process that uses these tools to minify and rename the development files and save them to a production directory.
Alternatively, the PageSpeed Module, integrates with an Apache or Nginx web server to automatically optimize your site, including resource minification.