Full site editing, often shortened to FSE, is the WordPress approach that lets you edit your entire site, including the header, the footer, and the layout of templates like your blog archive, using the same block editor you already use for individual pages. Before FSE, the content of a page was editable but the structure around it usually was not; changing the header or footer meant editing theme code or relying on whatever settings a theme happened to offer. FSE brings that structure into the same visual editor, so the whole site is built from blocks. It is a genuine shift in how WordPress themes work, and it is still maturing, since not every theme supports it and some established sites have good reasons to stay with the older approach. For a new build, full site editing can let a site owner adjust more of their own site without a developer, which is often exactly what they want. If you have heard the term "block theme," that is simply a theme built for full site editing.
Glossary entry