Minification is the process of code reduction without the sacrificing of its functionality. This became the industrial standart. The modified code files a taged with ".min" extension. But the PC perceives equally both minificated files and regular ones.
How to minificate the code
The minification can be applied by yourself with the help of online services similar to http://refresh-sf.com/.
There are a lot of specical programs for code minification. The best of them are https://github.com/fmarcia/UglifyCSS and https://github.com/mishoo/UglifyJS. They provide best compression according to the test results.
Also the minification is supported by several content management systems. And it can be done by individual plugins for such systems. For example, the Wordpress plugin https://wordpress.org/plugins/bwp-minify/ performs this.
The advantage of minification is in drastically reduce of page loading time. It's configured once. So be sure to take the time to it.