You are correct, but some knowledge about installing php, a webserver, a mailserver (for example the free hmailserver), etc is necessary to get it working.
If these things are saying nothing to you, then don't start with roundcube or hire somebody with the knowledge.
You have also squirrelmail, but RoundCube is much more advanced on the display and look. Squirrelmail has the advantage that it is an easy installation.