If you're not familiar with flechettes, that's understandable. The Washington Post reports the roughly inch-long finned darts "are rarely seen or used in modern conflict"—the Ukraine-Russia war apparently being an exception. The Post isn't presenting evidence that they've been widely used, but it visits with a woman who lives outside Kyiv in Bucha and who has piled up the flechettes that studded her yard. "If you look closely on the ground around my house, you will find a lot more of them," she says. Post reporters observed others in the streets.
The projectiles, or a version of them, were used in WWI and in Vietnam, and while they aren't banned under any international conventions (in part because their use fell out of fashion), groups like Amnesty International point out that they're indiscriminate weapons that should not be used in civilian-heavy areas. The Guardian in 2014 reported they were being used by Israel in Gaza and explained that when the shell containing the flechettes explodes, thousands of metal darts are dispersed "in a conical arch" nearly 1,000 feet long. A munitions expert tells the Post the weapon is most effective in specific scenarios, as when troops are massing together in an open space. (More Russia-Ukraine war stories.)