What is in-camera noise reduction…and is it worth it?
In-camera noise reduction will differ and be available or not from one camera model or manufacturer to another. However, the principle of this is the same throughout. It takes another exposure of the same duration (a dark frame, with the shutter closed) after your normal exposure finishes, and then applies it via a difference algorithm in the camera to the image, effectively doubling the amount of time each exposure takes to complete. Blegh.
As shown in the above picture, I always leave it off. I never use it. I don’t believe it does enough good for any photo I’ve ever taken, and I can do better noise reduction in post-production anyway.
The worst part about it, at least on the 5D Mark III (and II, and other Canons), is that when it applies the dark frame to the image, even the RAW, it cannot be undone, so in the end you have less control over your final product.