One of the more recently popular modules for Apache is mod_pagespeed. It is an output filter for Apache 2.2+ that can be configured through a variety of options through configuration files or a .htaccess file. An “output filter” is a something that transforms the data before it’s sent to the client. In other words, it’s a layer between your website and what the user’s browser receives when they visit your URL.
Before tablets and mobile phones, web pages were designed only for computer screens, and it was common for web pages to have a static design and a fixed size. Then, when we started surfing the internet using tablets and mobile phones, fixed size web pages were too large to fit the viewport. To fix this, browsers on those devices scaled down the entire web page to fit the screen.