What are tags used for you ask? Well it helps categorize your server and determine at a quick glance what it is.
Please note: Tags are used in conjunction with the filters. So if a server removed a specific tag from their filters they will not see that tag.
On the setup page of your server scrolling down a little you can setup tags by selecting tags from the "Tags" dropdown
For non premium servers, you can only pick 2 tags as opposed to 18 tags for premium.