Okay kiddo, so a fissile material cut-off treaty, or FMCT for short, is an agreement between countries to stop making certain materials, called fissile materials, that can be used to make really powerful bombs.
These materials are things like uranium and plutonium. They are very dangerous because they can release a lot of energy when they are used in a bomb. That's why countries want to be careful about who has access to them.
The FMCT is like an agreement between friends to not use certain toys, but it's on a global scale. It's a promise between countries to stop making these dangerous materials for weapons purposes. This treaty would help prevent countries from getting access to dangerous materials they could use for bombs.
So basically, the FMCT is a big deal because it tries to keep dangerous things out of the wrong hands, like a parent might hide medicine or bleach from you, so you don't accidentally hurt yourself or others.