Tags are the entities used to highlight some information about a checklist item. It could be it’s status (e.g. default one like IN PROGRESS or OPTIONAL) or absolutely custom one.
The benefit of using such is being able to use them in JQL Search or custom validations, or just for visual check in a consistent way.
Tags in Spreadsheet Editor is a comma-separated list:
The predefined OPTIONAL tag doesn’t affect the checklist progress:
