The block editor is the editing screen WordPress has shipped with since version 5.0. Instead of one text area, each paragraph, heading, image and embed is a separate block that can be moved, duplicated or styled on its own.
Why it replaced the classic editor
The classic editor stored everything as one blob of HTML. That worked for text but made layout awkward, which is why page builders became necessary in the first place. Blocks give structure to the content itself, so a theme can style a quote or a gallery consistently everywhere it appears.
Blocks and page builders together
A page built with a page builder does not use blocks; the builder takes over the whole editing screen. Posts, and any content type you have not handed to the builder, still use the block editor. Both can coexist on one site as long as you decide per content type which one owns it.