Full Site Editing is one of the recently added features in WordPress. It has been introduced in WordPress 5.9. FSE mainly focuses on the user editability of different site templates without making any changes in the code.
Full Site Editing is the umbrella project name for various sub-projects within Gutenberg:
The core concepts of FSE are:
Site Editor is used for building a site using blocks, layouts, templates, menus etc. Site Editor is supported in Block Themes. To Edit a block theme you can go to Appearance > Editor to open the Site Editor.