Tags link items together that have been assigned the same tag.
You can add a tag to an item in the right-hand information column or by dragging a citation (or multiple citations) over an exisiting tag in the left-hand column.
Up to 9 of yo turags can be assigned a color by right clicking on the tag and choosing Assign color.... Tags with colors assigned show a small square of that color in the middle column making these items easier to identify.
Some citations may import with automatic tags, especially when importing from library databases. To stop the automatic import of automatic tags by uncheck the box under Preferences General Miscelaneous that says Automatically tag items with keywords and subject headings.
You can also turn off the display of automatic tags and bulk delete them by choosing the Actions option on the lower right corner of the tags panel.
Delete any tag from your tags list by right clicking on the tag choosing Delete tag.... This will remove the tag from all citations to which it was linked.
Zotero also offers you the option to indicate that items within your library are related to one another. Create links by choosing an item in your collection and using the Related tab in the right-side column. When you click Add you'll see a search box that you to choose an item in your Zotero library for linking.