Noise Threshold means that when the difference per Pixel between 2 consecutive Samples is less than the threshold, Blender stops Rendering that Pixel. So some Pixels get less Samples than others, This is great for still images, but can cause issues in animations. Just as Denoising can cause problems in animations, although there are Denoisers, that actually take that into account; this is usually referred to as Temporal Denoiser.
