Full site editing (or FSE for short), is a collection of new WordPress features that focus on styling blocks and using blocks for all parts of a website, not only the content area. WordPress 5.8, released in July 2021, included the first support for full site editing with the introduction of the template editor.
Full Site Editing (or “FSE”) is a collection of interrelated features that unlocks the ability to edit your entire site using blocks. This allows you to take advantage of the familiar and flexible experience of the Block Editor in more places.