There are two ways of labelling your Knowledge Items: tags and keywords.
Tags show up in search and appear as labels at the bottom of items. You can add a tag to a knowledge item when creating or editing an item. You can manage which tags exist by going to Settings > Tags.
Tags are a good way to structure your content, ensuring that similar content is grouped together using consistent wording.
Keywords will be recognised in search, but will not be visible to users. They can be useful for including important words that appear in attachments, but do not occur frequently enough to warrant a tag.
Unlike tags, keywords cannot be configured from a settings page. Care should therefore be taken to make sure that they are consistent. Used sparingly, they are a useful tool when a tag isn't necessary, or when you don't wish users to see the keyword.