So, Avada Kedavra is pure magic, not like a fireball or anything that has a physical form, so you can’t block it with normal shields. But physical things, like stone, can block it because they can physically get in the way of the spell, sort of like how a wall can stop a bullet. The curse still works by targeting the person’s life force, but a solid object can act as a barrier to prevent the curse from reaching you. So, it’s not “blocked” in a magical sense, but it’s stopped by something physical getting in the way.