When you developing CSS for your website you will normally create your CSS in a way that makes it easier to read. This means each property will be on it's own line, you will have line breaks in-between classes and id's, you should even have comments between different sections of your styles.
It's easy to minimize your files you can either use a script to do this or make your IDE minimize your files.
But if all you have is the minimized file and you want to make some changes to this file it can be really hard to work out where your styles are, you need to un-minimize the file so you can see the styles in a readable way.
Here is a tool called CSS Beautifier which will allow you to enter minimized CSS and it will convert this into readable CSS for you. So if you only have the minimized file you can copy this into the tool and it will convert the entire contents to readable CSS.
Try it out.