Send it to your site and it'll tell you about what's making your site slow and how to fix it, and the list is in priority order so you know where to start.
Great for themes (e.g. picks up if you've forgotten to fully compress your images), but also good for testing your Moodle installation (e.g. it'll check if you've got gzip turned on).
PS I just created this exact post, word for word in the Themes forum, so if you want to talk about using this tool to make your Theme faster, you might want to go there instead: