What is JS compressing
This is a process of deleting unnecessary parts of JS code. Compacting JavaScript code can save many bytes of data and speed up your website.
How to minify JavaScript
Check whether your page needs JS compressing using the tool above. If the page has some unoptimized parts of JavaScript you will see similar message as on the picture below.

After getting this info, try to use tools recommended by Google, like JS minifier and Closure Compiler.