Tags are one or more key words associated to a given content on the site (post, page, blog, file, etc), enabling all the members of the site to find content that has been categorised by its members. The collected tags used across the site are are displayed in the right hand column organized by font size, where the bigger the size of the font, the more frequent that tag is used.
When you click on a tag in the side column it will take you to all the content in the site that has been tagged with that word (or those words).
You are always invited to add one or more tags to the content you create, separating each word or expression with commas.