Wireless charging uses magnetic induction to charge the iPhone. Don't put anything between the iPhone and the charger.
If a magnetic holder, case, or other object is placed between the iPhone and the charger, it may degrade the charger's performance or damage the magnetic strip or RFID chip (commonly found in some credit cards, security marks, passports, and smart keys).
If you have these sensitive items in your protective case, remove them before charging or make sure they are not between the back of the iPhone and the charger.