Wikipedia says minification is the process of removing all unnecessary characters from the source code of interpreted programming languages or markup languages without changing their functionality. CSS, HTML, and JavaScript Minifiers are free online applications to do the CSS, HTML, JavaScript code minification.
Minification is an optimization best practice that can deliver a significant website performance boost. Minifiers safely removes unnecessary characters, such as comments, whitespaces, and indentation. It leads to decreasing file size without affecting how the browser processes code. Minification dramatically improves site speed and accessibility, directly translating into a better user experience.