The reason there is a key in your mailbox is because you have a package that won't fit in your mailbox and the key is to a parcel locker or a larger mailbox depending on where you get your mail.
If you get your mail at a cluster mailbox such as on the curb then the key is for a larger part of the mailbox and the key will indicate the number of the mailbox that you use the key for.
If you get your mail at the post office then the key is for a parcel locker which is in the post office lobby close to where you get your mail from your post office box.
You simply look on the key ring for the number and then go to the large parcel locker and insert the key into the parcel locker and the turn it.
Then you can open the parcel locker and get your package.
The key will remain stuck in the parcel locker lock which is normal and nothing to worry about.
The postal worker will then use another key to eventually get the key back that was left in your mailbox.
The parcel locker is used when you have packages that are too large to fit in your mailbox and then you can pick them up at anytime.
If there's not enough parcel lockers available then the USPS will hold your package in the post office and leave a note in your mailbox to come in and pick up the package.