The Site Editor is where you create the templates and template parts that make up your Full Site Editing theme.
The main body of the Site Editor is the block editor. This includes the top toolbar and Settings panels.
There are some additional functions. Other functions aren’t available.
It’s similar to the block editor that you use to create content.
It enables you to edit both the site structure and (some of) the content.
It has some functionality that you won’t find when editing posts, or when editing templates.