It's the protocol that VSC made to speak to programs that do code analysis and is the basis of goto definition, autocomplete, refactorings etc.

It's used by most smaller editors so they can backpack off of the efforts languages make to be usable in VSC. (Vim, Emacs, Zed, etc)

Also diagnostics (errors, warnings), inlay hints like types and parameters, code lens (tiny embedded buttons), symbols, notifications like “document changed”, and more