By assigning a metadata set to an archive you control which metadata fields users may edit in that archive (assuming they are allowed to edit metadata in the archive in question, of course) and which taxonomies they will have access to.
What's more, users who are allowed to upload files to the archive will be required to fill in metadata in accordance with the field requirements specified in the metadata set.
Assigning a metadata set to an archive
Open up the archive properties in the site configuration and go to the Metadata tab. Then pick a metadata set from the drop down list. Click OK to close the archive details page and store the changes.