Late last week, Israel targeted three of Iran’s key nuclear facilities—Natanz, Isfahan and Fordow, killing several Iranian nuclear scientists. The facilities are heavily fortified and largely underground, and there are conflicting reports of how much damage has been done.

Natanz and Fordow are Iran’s uranium enrichment sites, and Isfahan provides the raw materials, so any damage to these sites would limit Iran’s ability to produce nuclear weapons.

But what exactly is uranium enrichment and why does it raise concerns?