What exactly does this mean? NIKON D600 lenses?

Compatible Format(s) FXDXFX in DX Crop Mode35mm Film

does that mean its only fx or fx and dx?

It's a full frame FX sensor. You can use the cropped sensor DX lens, but crop mode will prevent there being a black space around the image like there would be without the crop mode.

The D600 is a full frame DSLR, meaning it has a physically larger sensor than the typical consumer-grade DSLRs. Full Frame DSLRs have previously been considered professional grade, but the D600 is the first advanced amateur/prosumer DSLR.

In Nikon-speak, Fx = full frame. Dx = cropped.

A cropped camera is one having a physically smaller sensor. Nikon, Canon, Sony, and Pentax all have cropped and full-frame sensor cameras.

To use a larger sensor, you need full frame lenses. Full frane lenses can always be used on a full frame (Fx) OR cropped (Dx) DSLR. But this is not necessarily true the other way around (Dx lenses can't always be used on Fx cameras).

But unlike Canon DSLRs, Nikon Full Frame DSLRs can use either Fx or Dx lenses. Canon cropped lenses will damage a Canon full frame camera, and there's a pin on those lenses preventing them being mounted on the camera. Otherwise, the mirror of a Canon full frame camera will smack the back of the cropped lens. Poor design.

When a Nikon Dx lens is mounted on a Nikon Fx camera, it automatically goes into Dx mode (cropped) at 10MegaPixel resolution so that you do not get any vignetting in the photo (dark areas in the corners).

When a Fx lens is mounted, then the full resolution is available, and the camera behaves normally.

This allows you to upgrade to a full-frame camera and replace your lenses as you can afford to.

The Nikon D600 is a full-frame DSLR which means its sensor measures 24mm x 36mm - the same size as a 35mm film frame. Nikon calls this format FX.

Nikon DSLRs below the D600 - D3100, D3200, D5100, D90, D7000 and D300S - all have a sensor that measures 23.6mm x 15.6mm. Nikon calls this format DX.

A lens designed for a DX camera can be used on an FX camera but the camera will "crop" the sensor to 23.6mm x 15.6mm. The lens is still usable but you lose a large part of the FX full-frame sensor and, correspondingly, resolution.

However, this is an excellent design since it allows you to use lenses from your DX format Nikon on an FX format Nikon until you can replace your DX format lenses.