Usually, photography is a struggle to get enough light. Fast lenses, high ISOs, wide apertures: we do what we can. But it is sometimes a good idea to cut light.
Like when you want dark backgrounds and have plenty of flash power, or like when you want to create long exposures during the day, perhaps to capture a flowing waterfall.
The way to do this is to go to a low ISO. But once you have run out of low ISO, you need to use a filter. A neutral density filter (ND filter) is what you use.
Today’s Tip: if you do not have an ND filter handy, use your polarizer. This too cuts a couple of stops of light. That’s one good reason to always carry one for your popular lenses.