Overlapping instructions mean that when someone is doing something, they can also start doing something else before finishing the first thing. It's like when you're eating your lunch and talking with a friend at the same time. You can take a bite of your sandwich and talk about your favorite toy without stopping eating.
In computers, overlapping instructions happen when the computer is doing one task and at the same time is getting ready to do another task. The computer is like a very fast person who can do multiple things at once.
Imagine you are playing with your blocks, and your mom asks you to put away your toys in your toy box. You stop playing and put away your blocks. When you finish, you start putting away your other toys. But imagine if you could put away your blocks and toys at the same time, without having to stop one to start the other. That's what overlapping instructions do in computers, and it makes them work faster and more efficiently.