This one is a major upgrade to the game's visual fidelity (one of the big pieces of free content in 6.0 that's for everyone, not just those getting the Ancient Shadows expansion). There's also another hotfix to try to deal with the performance problems some people have been seeing since 5.042.
This is a standard update that you can download through the in-game updater itself, if you already have 4.000 or later. When you launch the game, you'll see the notice of the update having been found if you're connected to the Internet at the time. If you don't have 4.000 or later, you can download that here.