The Compare tool allows you to compare several items in a horizontal row. If you're on desktop, consider using the Organizer for more powerful comparison.
The Compare menu uses a coloring gradient to compare stat values. Colors closer to the left are better while blue means "best."
The item you opened Compare from is highlighted with a gold title.
You can click the "Open in Organizer" button to open the same items in the Organizer, which has more columns and sorting options.