Tags are like keywords you can apply to a page. They can assist people to search for specific content on a wiki.
A page about cats and dogs for example, could be tagged 'cat' dog' 'pet' 'animal' and so on
You do not have to apply tags to your pages.

How do I add or remove a tag?

To view your tags click on the Details and Tags option under PAGE in the top right corner of the page
You will now see your tags listed at the top of the page.
To add (or delete) a tag for a page simply click the EDIT button next to the TAGS information at the top of your page in viewing mode.

A full list of site tags can be found via Manage wiki - Tags