This test will check if your domain(and ultimately the server you are hosted on) has GZIP enabled by connecting to your domain and requesting the required information. Next to that it will also give you some additional GZIP information like the size of the GZipped version of your URL(Gzipped Size(compressed):) and how much you (may) benefit from GZIP compression(Compression %:).
GZIP compression makes it possible to compress your web files (HTML files, etc..), sometimes up to 70%-80%. This much smaller compressed version of your files is then sent to the browser of your visitor instead of the larger original files. The browser of your visitor will then automatically extract the compressed file and show the full page to your visitors.
The GZipped file is much smaller in size (in bytes) as your original web file. The browser of your visitor will need less time to load your website because it needs to download much smaller files.
Go here for a detailed guide how to enable GZIP and for more information about GZIP in general.
Analyze your CSS to make improvements to the CSS Delivery.
Minimize CSS scripts and files for page speed.
Analyzes the Javascript delivery of your pages.
Minify your JS files and scripts to maximize performance.
Check whether your pages have Gzip compression enabled.
Tests your image delivery so you can improve it.
Reduce your PNG image filesize while keeping quality.
Play with the quality and size of JPEGs to save disk space.
Place multiple images in one image to make fewer requests.
Check how and whether your website files are cached.
Test your web pages for broken links and requests.
How many HTTP requests do your pages produce?
Encode JPEG and other files into a Base64 string.
Check whether your website has keep-alive settings as enabled.
Remove line breaks from your scripts to save disk space.
Check the HTTP server header of your site.