modelines allow you to put a special comment in your file (typically on the first or last line of the file, and [for vim] of the format `# vim: set ts=2:`) to configure a subset of editor settings automatically whenever that file is opened in a buffer. They're very common, especially in codebases where there are lots of different styles in different files. Many editors (e.g., Zed (as of https://github.com/zed-industries/zed/pull/49267)) support vim and/or emacs's syntaxes for modelines.
Modelines were disabled by default for security reasons for a long time. It's kind of wild to me that they're enabled in some distributions, but there still isn't much restriction on what settings can be configured; I've never seen a modeline in the wild that did anything other than set `filetype`, `fileencoding`, `tabwidth`, `expandtab` (hard tabs vs spaces), and maybe `tabstop` / `softtabstop`.
Modelines were disabled by default for security reasons for a long time. It's kind of wild to me that they're enabled in some distributions, but there still isn't much restriction on what settings can be configured; I've never seen a modeline in the wild that did anything other than set `filetype`, `fileencoding`, `tabwidth`, `expandtab` (hard tabs vs spaces), and maybe `tabstop` / `softtabstop`.