If you click the "Why LSP?" link in the first sentence of the article, or type "lsp" into Google, you'll get your answer:
> The Language Server Protocol (LSP) defines the protocol used between an editor or IDE and a language server that provides language features like auto complete, go to definition, find all references etc.
> The Language Server Protocol (LSP) defines the protocol used between an editor or IDE and a language server that provides language features like auto complete, go to definition, find all references etc.