So, imagine you have two toys - a teddy bear and a toy car. They are both fun to play with separately, but one day you want to make them work together. You take a piece of string and tie it to the car, and then tie the other end to the bear. Now, when you move the car, the bear comes along for the ride, and when you move the bear, the car follows him.
That's kind of like cross-application scripting. Different applications (like toys) can work together if they are tied together or connected in some way (like with string). And just like you can control both the bear and the car by moving one, cross-application scripting lets you control different apps (like websites, programs, or games) by using features from one app to interact with another.
For example, let's say you are playing a game that asks for your name. Normally, you would have to type your name in yourself. But with cross-application scripting, a different app (like a program you made yourself) could send the game your name automatically. This lets you do things with apps that they weren't designed to do by themselves.
Of course, just like playing with toys tied together with string can sometimes be tricky, cross-application scripting can sometimes have problems too. Some apps might not work well with others, or could be dangerous if they communicate with each other in the wrong way. So it's important to be careful and make sure you understand what you are doing when you try cross-application scripting.