Okay kiddo, so sometimes grown-ups work for the government, like being the boss of a big group of people who make important decisions together. This group of important people is called a cabinet.
Now imagine that one of the grown-ups in the cabinet decides they don't want to be part of it anymore. They want to quit their job or take a break from it. This is called resigning or stepping down.
When one of the grown-ups in the cabinet decides to resign, that means they are leaving their job as part of the cabinet. And when more than one person from the cabinet resigns at the same time, it's called a demissionary cabinet. This means that the cabinet is not complete anymore, and there are fewer people to make important decisions together.
It's kind of like if you were playing a game with your friends, and some of them suddenly decided they didn't want to play anymore. The game won't work as well without all the players, and it might be harder to make decisions or come up with a plan.
So when a cabinet becomes demissionary, it means that things might change a lot in the government. New people might be chosen to replace the ones who left, and they'll have to work together to keep making important decisions and running the country.