When automotive parts are manufactured, there are components that can later be recycled even when the rest of the part no longer works. These recyclable parts are called the “core” and suppliers will generally include a deposit on them when a new or remanufactured part is sold.
If you have bought a part to replace one that no longer works and if there is a core in this part, you will probably have to pay a core charge when you buy the replacement part. The core charge is like a deposit, and if you bring back the core from the original part, you will recieve your deposit back. It’s that simple.
Core charges are used as a way to encourage recycling. Just like the deposit you might pay on a tin can for a drink will encourage you to return the can to a recycling center to get your deposit back, so too will a core charge encourage you to return the recyclable component of a part so that it can also be reused. If the original core was so damaged that it is no longer useful, then you won’t get your core back. But if it is still in good working condition and meets the shop’s criteria, then you should be fine.
And remember: the core deposit is paid for with your money. It might require some extra effort to bring it to the shop, but it is financially worth it in the end.