Explain Storage Credential and External locations used in conjunction with Unity Catalog to a five year old.

Sure, Unity Catalog is a service, just like any other service we use in our daily lives, such as going to a car wash station to get our car washed. Similarly, Unity Catalog provides a service that helps protect data assets and gives the owner of those assets the power to decide who they want to provide access to see these assets.
Storage Credential is like a safe with a unique key. If anyone wants to borrow something from the safe, they have to provide that key to open the safe. Generally, safe owners don't want this key to be given to everyone, so they create external locations specific to everyone's needs. Storage credential owners can create multiple safes and distribute their keys instead. So, in summary, Storage Credential and External Locations work together to unlock a secret.