The haskell-language-server (HLS) project is an implementation of a server (a
"language server") for the Language Server Protocol (LSP). A language server
talks to a client (typically an editor), which can ask the server to perform
various operations, such as reporting errors or providing code completions. The
advantage of this system is that clients and servers can interoperate more
easily so long as they all speak the LSP protocol. In the case of HLS, that
means that it can be used with many different editors, since editor support for
the LSP protocol is now widespread.

HLS is responsible for actually understanding your project and answering the
questions that the client asks of it, such as: what completion items could go
here? are there any errors in the project? and so on. HLS provides many (but not
all) of the features that the LSP protocol supports.

But HLS only provides the server part of the setup. In order to actually use it
you also need a client (editor). The client is responsible for managing your
interaction with the server: launching it, dispatching commands to it, and
displaying or implementing responses. Some clients will even install the server
binaries for you!