Everything the user it signs in as may do, through the same screens a person uses. The admin describes itself at /admin/ai-guide: the screens that user may open, every content type with its fields and limits, and how saving, autosave, translations and media work.
In practice an agent writes and translates entries, uploads pictures and sets their alt texts, edits menus and site settings, and reads the messages of the contact form. It cannot do more than its user: an author's agent edits only that author's drafts. Every change goes into the activity history with the user's name.