This integration lets you send files to an Element/Matrix conversation directly from Nextcloud Files.
The account configuration happens in the "Connected accounts" user settings section. It requires to get a personal access token in your Element settings.
A link to the "Connected accounts" user settings section will be displayed in the widget for users who didn't configure an Element/Matrix account.
There also is a "Connected accounts" admin settings section if you want to allow your Nextcloud users to use OAuth to authenticate to a specific Matrix instance.