Classic Editor by WordPress Contributors disables the newer “blocks” page editor in favor of the word-processor-like older editor, TinyMCE. There’s also an option to let the user choose on a per page basis which editor to use, and which is the default.
This is an extremely popular plugin because a lot of people hate the Block editor and prefer something that’s simpler to use even if not so feature-rich. Depending what you’re doing, that might be okay, but please read about why you should stop worrying and love Blocks before you install this. I used to prefer the Classic editor myself, but Blocks has gotten a lot better since it was introduced.
So: valuable plugin for some, but I encourage you to take off the training wheels.