CSS minification is the process of removing unnecessary characters and formatting from a CSS stylesheet in order to reduce its file size and improve website loading speed. A CSS minifier is a tool that automates this process by parsing the CSS code and removing all extraneous whitespace, comments, and other non-essential characters.
As you can see, the minified version is much more condensed and easier to read by a machine, while still retaining the same functionality.
There are many CSS minifiers available online that you can use for free, such as the YUI Compressor, the Closure Stylesheets Compiler, and the CSSNano. You can also find plugins and extensions for popular text editors and integrated development environments (IDEs) that will minify your CSS code automatically as you work.