Okay, so you know how different churches have different rules and special people in charge called priests? Well, the military also have special people who work as priests for soldiers who need spiritual guidance and prayer.
The military ordinariate is a special part of the church that handles everything related to this - from finding priests who can work with soldiers to making sure there are chapels for them to go to.
It's like a special team that makes sure soldiers who want to talk to a priest can find one easily, even if they are out in the field or in another country. They also make sure that these priests are trained to deal with the unique challenges of being a soldier, like being away from family or in dangerous situations.
So, in short, the military ordinariate is like a special church department that helps soldiers connect with priests and offers them support and comfort during difficult times.