Literally changing the editor code, to do whatever you want. Emacs is a lisp (elisp) environment mostly with a small C based core for performance sensitive primitives. But the user code is at the same "level" as the Emacs core code.

This is very different from most extensible editors, where extensions are sandboxed and have a limited, controlled API to the core editor, which is kept separate.

This makes Emacs very "malleable", and it's likely easier than with most other tool to change it in custom, specific ways. It may seem dangerous, but it comes from a culture where this extensibility is very front and center, with documented design pattern to support it (hooks, customization settings...).