Achromatopsia is a fancy word that means someone has trouble seeing colors. You know how you can see yellow flowers and green grass and a blue sky? Well, someone with achromatopsia has a hard time telling those colors apart.
It's like looking at everything through a black and white movie camera. They see things as shades of grey instead of bright and colorful like you do.
This condition means they miss out on all the beautiful colors around us, but it's not something to be afraid of. Special glasses or contacts can help make colors pop out more for those who have achromatopsia.