The Documents (Atlas) web part is a pre-configured In Focus web part which appears by default on a number of Atlas page templates showing the latest documents relevant to the page. On some page templates which include this web part it may be named "Latest Documents" showing the most recent Knowledge articles in the current area.
The screenshot shown below is using the default layout "Carousel":
Each tile shows the name and date of the document, the file icon and a preview image of the document. Users can click the title or image to open the document, or use the arrows in the top right to page through the available documents.
You can click the ellipsis (...) in the bottom right to see additional options:
Documents can be uploaded via the Add It wizard. See how here: Add a Document or Email using Add It
Edit or Delete existing Documents
To Edit or Delete Atlas content please check this article: Edit or Delete Atlas content
You can customise the layout and query of a Documents web part by editing the page, then editing the web part and modifying the configuration options. All the configuration options for In Focus web parts are described in the linked article: Configuration for In Focus web parts