Tags can be associated with an Activity, just as they can be with an Item.
In the Author Site, tags are created for Activities in the same way they are created for Items - through the Tag Manager interface.
To tag an Activity, open the Author Site Activity Editor, and click on the Tags tab. Start typing your tag, and hit enter on your keyboard when you find the tag you're looking for. Click save in the top right corner and you have now added a tag to your Activity.
You can also add and update tags on an Activity using the Author Site bulk updates feature.