The Language Server Protocol is essentially a standard for editors to talk to language tooling. All the usual refactor/go-to-definition/auto-complete functionality you expect from a modern IDE is implemented in a language-agnostic way in the editor in terms of calls to the language server, and language servers then provide the actual concrete implementations of that functionality.

IIRC Microsoft defined the spec in the context of creating VS Code, but these days every editor and their uncle supports it (from Vim and Emacs to Sublime Text and even IntelliJ). You can get a solid IDE-like experience for most mainstream-ish languages, and on most editors, through LSP.