Language servers provide generic autocomplete and other language-specific functionality (e.g. rename a method, find all references to a symbol within your project, find where a symbol is defined, etc) to any editor that supports them. The reference implementation is VSCode, so if you use VSCode you can definitely use language servers, but now that’s it’s picking up steam, support is getting added to other editors, e.g. see vim-coc.
The Ruby language server in question: https://solargraph.org/