compression

Compression is a way to decrease your web pages and css files size in order to improve page loading speed. This change is performed at server side, before sending the file to browser for reading, e.g. index.html file is converted to index.html.gzip. Apache servers have two modules available to decrease files size for this purpose, mod_deflate – that’s a quick and easy to set up standard one and mod_gzip that has more advanced functionality available, for example, pre-compressing.

Feel free to check for more details in Google Developers guidance. You can also check here if your website has GZip compression enabled.