A key is obviously a symbol for opening or closing.  It can symbolize opening or closing a door, a lock, a treasure chest, or a pathway.  In religion and spirituality, it is generally symbolic of opening the path to enlightenment as in the phrase: "Keys to the Kingdom".  Since it is spiritual knowledge - gnosis - which opens that path, a key is also a symbol of knowledge or gnosis.  A silver key is said to represent opening temporal, or lesser powers, while a gold key opens the greater, or spiritual powers.  In Christianity, a key is a symbol of St. Peter as the guardian of the Pearly Gates.   The Bible references the "key of David" in several places and John states that Christ is the one who holds the key meaning that Christ has the key for access to Heaven or the Kingdom of God.
    On the U.S. one dollar bill, you will find the seal of the treasury.  Within it is a key which symbolizes the authority of the treasury.
In fairy tales, a key often represents a mystery to be solved.