Okay, let's imagine you have a toy box with lots of toys in it. But instead of just being a normal box where the toys are all mixed up, let's say you have two separate compartments in the box where you can put toys. But you want to be able to figure out how many toys are in each compartment without actually looking inside.
So, you come up with a plan. You take a piece of paper and write down the number of toys in each compartment at the beginning. Then, you start playing with the toys and moving some of them from one compartment to the other. After a while, you stop playing and write down the new number of toys in each compartment.
Now, here's the tricky part. Even though you moved toys from one compartment to the other, you still want to be able to figure out how many toys are in each compartment without actually looking inside. So, you use something called the nonequilibrium partition identity. This is a fancy way of saying that you can use some math to figure out how many toys are in each compartment based on how many you started with and how many you moved between compartments.
It might sound complicated, but think of it like this - if you started with 10 toys in one compartment and 5 in the other, and then you moved 2 toys from the first compartment to the second, you can use the nonequilibrium partition identity to figure out that you now have 8 toys in the first compartment and 7 in the second. And you can do all of this without actually looking inside the box!
In science and engineering, the nonequilibrium partition identity is used to help understand how different processes and systems work. It allows us to make predictions and calculations based on what we know about the system, without having to directly observe it. It's kind of like using math to solve a puzzle or play a game - you might not be able to see all the pieces, but you can figure out what's going on based on the ones you can see.