That means you'll never lose your changes anymore.
You can see all the improvements you've made, and easily copy or export them all.
To see all the changes, click on the button "CSS Changes" on the top bar:
A modal (popup) will appear:
There are 3 options on the top:
Export as a file
Export all changes as a .css file
Reset all changes
Reset all changes (un-reversible). The CSS will back to its original version.
Copy all changes
Copy all changes to the clipboard
And then each modified element is shown separately with a git diff visualizer (see which lines you removed, edited, or added), and you have 2 options:
Copy changes only (copy only the changes to the clipboard)
Copy new code (copies all the CSS code for that element to the clipboard)