Simple definition
Full Site Editing lets administrators customize WordPress templates and global site elements using blocks.
Purpose and use
Full Site Editing names capabilities extending block editing into templates and other site-wide parts.
How does it work?
The Site Editor, global styles, templates and template parts support work beyond one post's content.
Example and verification
Before a global change, identify whether the edited object is a page, its template or a shared part such as the header.
Points to watch
Scope depends on the edited object and compatible theme. Changing a template can affect every page using it.
In short
These capabilities provide broader site control and require corresponding care with shared structures.