Neutral density (ND) filters typically reduce the intensity of light, without affecting color or tone. The primary use of a ND filter is to enable slow shutter speeds (or low apertures) in bright light. For example, if you want to blur the motion of water (a waterfall or river) under daylight; this would be impossible without a neutral density filter.
Most ND filters will also work in infrared light, just make sure it’s made of glass as plastic and resin filters tend to only work in visible light and pass IR light unaffected. Just take a picture of a person with sunglasses on and you will see that in IR those sunglasses become transparent.