Tags are a way to filter the contents of a site. Each page can be assigned Tags: right-click a page in the Pages menu and go to Settings. Here, enter one or more tags (separated by commas).
Linking to Tags
Once you’ve added Tags to your pages, you can add links to filter the contents of your site. For example, if you have a Pinned navigation, you could add text links like this: Tag 1, Tag 2, Tag 3 — select one of them and open the link menu, go to Navigate... and then Tags... Here you can enter which Tag(s) to link to. These should correspond exactly to the tags you’ve added to your pages. When a tag link is clicked, the pages that contain the tag(s) will be displayed in a vertical scroll. Linking to multiple tags will return pages that contain any of these.
Thumbnail Index
Another use of Tags is when when displaying a Thumbnail Index — these can be configured to return thumbnails of pages with specific tags: in the “Add...” menu in the top menu bar, select Thumbnail Index, then after the index is added, click on the red Thumbnail Index button to open the settings, and filter it by Tag.