Tags contain information about items, pages, etc., but are not standardized like metadata is. You can think of them as a way for folks to find items in your site (like keywords) and see how items or exhibits relate to one another. Collaborate with your colleagues about how you'd like others to discover related items.
In Omeka, you can add tags to items and to exhibits. When adding tags to an item, make sure to click "Add Tags" before clicking "Add Item" or "Save Changes." If you do not click "Add Tags," the tags will not be attached to your item and will not be saved.
Once you've added tags, you can view and edit your tags by clicking "Tags" in the left sidebar.