A restocking fee is money kept back from your refund when you return an item. It is deducted from what you get, so a return that looks free can still cost you.
It usually applies to a category, not the whole store
Very few retailers charge a restocking fee on everything. The fee typically targets goods that lose value once opened or that are expensive to handle: large appliances, televisions, computers, drones and cameras, custom or made to order items, and anything that has to be shipped freight.
Opened, and unopened, are different transactions
Many policies charge nothing if the item comes back sealed and charge a percentage if the box has been opened. Where that applies, opening a package to look at something is the decision that costs you money, not the return itself.
Fees you are usually not charged
Restocking fees generally do not apply when the retailer was at fault. If the item arrived damaged, was faulty, or was not what was described, the policy normally waives the fee. Retailers state this explicitly, so it is worth quoting back to them if a fee appears anyway.
What to check before you buy
For any large or high value purchase, read the fee percentage and the exact category wording on the store page, then decide whether to open the box or return it sealed.