What is gzip compression?
When a user hits your website a call is made to your server to deliver the requested files.
The bigger these files are the longer it's going to take for them to get to your browser and appear on the screen.
Gzip compresses your webpages and style sheets before sending them over to the browser. This drastically reduces transfer time since the files are much smaller.
In terms of cost versus benefit, gzip compression should be near the top of your page speed optimizations if you don't have it setup already.
How does it work?
Gzip is actually a fairly simple idea that is extremely powerful when put to good use. Gzip locates similar strings within a text file and replaces those strings temporarily to make the overall file size smaller.
The reason gzip works so well in a web environment is because CSS files and HTML files use a lot of repeated text and have loads of whitespace. Since gzip compresses common strings, this can reduce the size of pages and style sheets by up to 70%!
Gzip has to be enabled on your webserver which is relatively straight forward.
When a browser visits a webserver it checks to see if the server has gzip enabled and requests the webpage. If it's enabled it receives the gzip file which is significantly smaller and if it isn't, it still receives the page, only the uncompressed version which is much larger.
Why Is It Important
The main reason it is important is because it reduces the time it takes for a website to transfer the page files and style sheets which ultimately reduces the load time of your website.
How To Add Gzip
There are different methods of setting up gzip compression depending on whether or not you've got an IIS or Apache server (or something else entirely).
If your server is IIS, follow these instructions in the Microsoft TechNet document to enable compression.
You will need to add the following lines to your .htaccess file:
AddOutputFilterByType DEFLATE application/rss+xml
AddOutputFilterByType DEFLATE application/vnd.ms-fontobject
AddOutputFilterByType DEFLATE application/x-font
AddOutputFilterByType DEFLATE application/x-font-opentype
AddOutputFilterByType DEFLATE application/x-font-otf
AddOutputFilterByType DEFLATE application/x-font-truetype
AddOutputFilterByType DEFLATE application/x-font-ttf
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE font/opentype
AddOutputFilterByType DEFLATE font/otf
AddOutputFilterByType DEFLATE font/ttf
AddOutputFilterByType DEFLATE image/svg+xml
AddOutputFilterByType DEFLATE image/x-icon
AddOutputFilterByType DEFLATE text/css
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE text/xml
# Remove browser bugs (only needed for really old browsers)
BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4\.0 no-gzip
BrowserMatch \bMSIE !no-gzip !gzip-only-text/html
Header append Vary User-Agent
After you've saved your .htaccess file, test your site again in GTmetrix.com to make sure it has been properly compressed.