Flat-File CMS
Also known as: file based CMS, databaseless CMS
A content system that keeps pages in ordinary files on the server instead of in a database.
Normally a website keeps its words in a database, which is a separate program the site talks to. A flat-file system skips that and keeps each page as a file you can open and read.
That sounds like a small technical detail and it changes daily operations. A backup becomes a folder copy. Moving a site becomes a folder copy. Handing it to a client becomes a folder copy.
The trade is real. Databases are genuinely better at some jobs, like a store with thousands of products or a site where fifty people search and filter at once.
For the kind of sites a small shop builds for local businesses and professionals, the file version removes more problems than it creates.