File Tags allow you to organize your files and more easily filter your list of files to just those that fit specific criteria. Any user can apply existing tags to a file, but only admins can create new tags.
To apply a tag from a company, deal, or person profile page, simply click within the cell under Tags for the file you wish to apply a tag to and enter in the name of the tag you wish to apply. Autocomplete options will show for you based on the tags your admin has set up.
You can also add tags from the Files list view page in a similar manner by clicking on the appropriate cell under the Tags column and entering in the tag you wish to apply.
If you're an admin who needs to learn how to add, manage, and remove tags, you can learn more here.