Prompt global strike is when a country (like the United States) wants to quickly attack a target far away. It's like when you throw a ball really really hard and really really far. But instead of a ball, it's a bomb or missile that can go really really far and hit a specific target.
Sometimes, countries need to act fast if they think there is a bad guy who might do something really bad. But sometimes it takes a long time to move big weapons (like planes) across the world. So prompt global strike is a way to quickly hit a faraway target without having to move a big army or weapons across the world.
It's important to remember that prompt global strike can be dangerous and needs to be used carefully because it can hurt a lot of people. So countries need to think carefully before using it.