A CSS Minifier is a tool that reduces the size of CSS files by removing unnecessary characters, such as white space, comments, and new lines, without changing the functionality or appearance of the website. The process of minification can significantly reduce the file size of CSS files, which can improve website loading time and reduce bandwidth usage.

CSS minifier tools work by analyzing the CSS code and removing any redundant characters, including spaces, tabs, line breaks, and comments. They also remove unused or redundant selectors and properties. These tools often provide additional features such as CSS syntax validation and error detection.

There are many CSS minifier tools available, both online and offline. Some popular examples include:

• Online tools: CSS Minifier, CSS Compressor, MinifyCSS
• Offline tools: YUI Compressor, CSS Nano, Clean-CSS

Using a CSS minifier can help optimize your website's performance by reducing the size of your CSS files. It's important to note that while minification can make your code more difficult to read and maintain, it doesn't affect the functionality or appearance of your website.