Imagine you have a toy that belongs to someone else, and you know it's not right for you to have it. You might try to sneakily sell that toy to someone else to make some money. That person who buys it from you might not know that it's stolen, and they might even go on to sell it to someone else after you.
A fence is a person who buys and sells stolen items like that toy, but they do it on a bigger scale, not just with toys. They might sell things like jewelry, electronics, or even cars.
While it might sound like a good way to make money, it's actually illegal and can be very dangerous. Fences are called "fences" because they "fence" stolen goods away from the rightful owners. Being a fence means that you're part of a criminal operation that not only steals things, but also contributes to more crime and harm.