Hmm, maybe its only a US thing? By the power of Google I summon the definition as provided by an auto parts store website:
What is a Core Charge?
A “Core Charge” is similar to the deposit you might pay for a can or bottle of soda. In many states, to promote recycling, you are charged a deposit when you purchase a can of soda and you receive your deposit back when you return the empty can. Many automotive parts have a Core Charge, or Core Price, that works the same way as a soda can deposit.
Here’s how the Core Charge works:
The part you are purchasing contains a component that is recyclable
The Core Charge deposit is charged at the time of purchase
When the recyclable component from your old part is returned to us, the charge is refunded to you
To receive your Core Charge refund, you can do either of the following:
Bring the part to any Advance Auto Parts store along with your receipt or packing slip
Mail the part back to Advance Auto Parts following the directions including on your packing slip.
Lots of items offer a core charge return (batteries, alternators, starters, etc...).