Minifying CSS takes the beautified, well formed CSS code that you have written and removes the spacing, indentation, newlines, and comments.
When do you use?
These elements are not required for CSS to be used successfully. It also makes the CSS more difficult to read. The 'best practice' of many developers is to maintain a 'beautified' version, and when rolling out their project will run the styles through a minification program. They will also combine their many style files into one file.