Don't worry. One of the frustrations of IT is that the same words get re-used to mean different things.
In hardware a server is a computer that you use to run backend processes, such as databases. These usually are built to higher resilient specs.
A server can ALSO be a piece of software that serves up data, such as a stream of music; this is used/consumed by a software client.
Tidal will allow you to create a username & password to access their music, and then stream that over the internet to you. This might be to your browser (e.g. IE or Firefox), but in your case you will want to stream it to your Rendu. To do this you need some intermediary software to:
Log on to Tidal; and
Serve up the music stream in a way that can be consumed by the Rendu.
This is the 'Server' .....which in my case is LMS.
Hope this is less opaque than I fear it is!