Look up HTML5, CSS3, etc features, know if they are ready for use, and if so find out how you should use them – with polyfills, fallbacks or as they are.
Remembering what CSS to prefix is hard. Today browsers develop quickly (yay) and things often change (boo). Preprocessors and mixin libraries are not always up to date.
This page has one purpose: simply show what prefixes are needed for a newer CSS property. Need more information? Check the awesome Can I Use or Autoprefixer!