Terminals can't send certain modified keys, something like M-S-;. What this thig does:

- Uniformly describes different key codes across different terminal emulators and platforms;

- Sends the codes which native platforms support, but the terminal protocol does not, via a custom prefix. It gets decoded on the Emacs side, and mapped back to a native-matching key code. You can press fancy keys and have a uniform reaction in Emacs, no matter if you're using a local graphical Emacs, or remote Emacs in a terminal.

I confirm, this is a great approach; I used a much simplified version of it with WezTerm and remote Emacs.