To keep your references organised, Zotero offers Collections and Subcollections.
To create new Collections, click the small folder icon at the top left of the menu bar 
or right-click on My Library and choose New Collection: 
Tags provide a quick way to mark references with similar information, such as themes or authors. Zotero allows you to create your own custom tags, and assign as many tags as needed to your references.
Up to 6 tags can be given more importance by being assigned a number (1-6) and colour.
Tags are visible in the right-hand panel and shown against references assigned coloured tags, e.g.:
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