Enabling compression is one possible method to reduce the size of HTML files and assets like images. Which in return will have a positive effect on the loading time. There are various ways of compressing a site. We distinguish between gzip and deflate compression.
Compression is for advanced users. A webmaster is able to active compression on the server. It is very well known in combination with Apache Servers and it's really important to improve your page speed.
Learn more about compression:
Google about Compression
Guide to enable Compression