A Language Server is a library that provides language-specific features for programming languages such as code completion, syntax highlighting, and error checking.
Language Servers are implemented with the Language Server Protocol (LSP). LSP was developed in collaboration with Microsoft, Red Hat, and Codenvy.
https://en.wikipedia.org/wiki/Language_Server_Protocol https://microsoft.github.io/language-server-protocol/
A book about writing Language Servers:
Language Server Protocol and Implementation: Supporting Language-Smart Editing and Programming Tools
https://www.amazon.com/Language-Server-Protocol-Implementation-Language-Smart/dp/1484277910