Tags are a way to organize items outside of the outline hiearchy. You can filter by tag, allowing you to create different views of your outline.
This is what tags look like:
To assign tags to items:
In this example I'm adding a "#today" tag:
If there are a tags that you use frequently you can setup keyboard shortcuts for them. That way you can apply them without needing to use the tag editor.
To filter your outline by tag:
#to find all items that have any tags at all.