Whenever a user connects to your website, the server sends multiple files to their computer. The smaller these files are, and the fewer of these files there are in total, the faster they can be transmitted. Minifying (a means of compression) and concatenating (a means of combining) your files allows them to be transferred much more quickly and efficiently, which can greatly improve the user experience and performance of your website.
CSS is minified in 3 main ways:
By doing this, the size of a CSS file can easily shrink by 20%, sometimes more.
The most sensible and maintainable way to minify your CSS is by using some sort of automated task, usually through a task runner like Gulp or Grunt. These allow you to "watch" files for changes, and automatically minify your CSS into a separate file whenever that file changes. You then point to the minfied file in your HTML (not the original), allowing you to always have a copy of both your original working CSS, and the minified production copy. Popular CSS minifiers include gulp-cssmin and grunt-css.
If for whatever reason you can't or don't want to use a task runner, there are also a number of copy/paste online CSS minifiers available. Just be sure to save your original formatted CSS in case you need to make changes later.
As above, a task runner is the optimal way to concatenate your CSS files as this will allow you to automatically concatenate files in a particular order, and then load those into your HTML document. This can be done with gulp-concat-css or grunt-concat-css.