If you want to apply the same tag to all content on a particular topic, you can set your account to automatically tag each mention as you save it. In the below example, I am creating an 'Education' topic and specifying that whenever I save any mentions that match the search terms from this topic the tag 'Education' will automatically be applied.
First I create my search and save the search as a topic:
In saving the topic, I select the option to 'Tag Mentions Automatically on Save' and create or apply the tag 'Education'.
From now on, whenever I save any mentions that match the search terms of this topic, the Education tag will be applied.
From within the topic, you can manage the tags, by selecting the icon shown in the right-hand panel indicated below:
You can also manage your tags by going to ALL SAVED COVERAGE in Saved Coverage / Folders. The Manage tags icon is again in the right-hand panel and allows you to create, edit or delete tags, including auto-tags.
A full explanation of how to use folders and tags to organise your mentions is here.