A graphics client is what takes the data computed by the Orbiter core and renders graphics. In short, the graphics client is what you see while running Orbiter.
By default, Orbiter uses an inbuilt DX7 graphics client. For the new version, Artlav has been working on an OGLA client as an alternative, which is packaged with the betas. You can opt to use this client by firing up the orbiter_ng.exe and activating its module, then configuring it on the video options page.