There's more to Engineering than just building new things
Sometimes Engineers have to analyze something that was built before, either to learn about it, or to understand why it broke. Sometimes the person who built something is not around - but you need to understand how it works. Maybe it stopped working, and you need to fix it. First, you need to understand how it's supposed to work before you can fix it. Or, maybe you want to improve it so that it can do more things. Then, you need to understand how it works so that you can add new capabilities to it.
Reverse Engineering is where Engineers take apart an already-built device to understand what it does and how it works
Why would someone want to take something apart? Well, there are a few possible reasons. One might be just to learn about it and understand how it works.
Imagine if you were to take apart your television… (Please don't actually do that, your parents might get mad at you!) But, if you did, you could see what the components (parts) are.
Electronic components tend to have labels – called part numbers
– on them to identify what they are. You can look up the components to identify their function. (You can look up just about anything online these days.) If you identify the parts and what they do, you can get an understanding of how the thing you use every day actually works. Engineering students often take apart devices for these very reasons. If you have older, broken devices hanging around your house (anything from mixers to remote controls to toasters), you might think about (safe!) ways to do a little of your own reverse engineering. But, please --- only do this with an adult's permission, and with competent adult supervision!
Sometimes people want to Reverse Engineer something built by another group of people, so that they can imitate it
Imagine if an alien spacecraft landed in your backyard. Wouldn't you want to know how their spacecraft works, and whether they know something we don't about space travel? Or, imagine that a well-known company is working on its next generation smartphone. Suppose that everyone wants to find out about what it will look like, what it can do, and what great new capabilities it will have. Imagine that an engineer in the company has a prototype (early example) phone and has brought it along on a night out on the town. And, imagine the engineer somehow left it behind in a bar or restaurant. You can bet that the phone will find its way into the hands of someone who will do their best to reverse engineer everything they can to find all about the new phone. As a matter of fact, you might recognize this example as an actual news item! The actual news item was about a to-be-released Apple iPhone.