You can't open a door without  the key! There are all sorts of of examples of symbolism and metaphor around keys. Just to name a few:
  • The top center stone of an arch is the keystone
  • Island chains have keys in their names like the Florida Keys
  • One plays the keys on the piano
  • A musical scale is sometimes referred to as a key
Keys are most often symbols of knowledge or wisdom. One most know something before being granted the power or freedom behind the door. We often give people "key points" to remember when teaching them something new or giving new information. Movies and books involving keys are: The Indian & the Cupboard, The Skeleton Key, in The Matrix: Reloaded there was a key master the heroes were trying to protect. The Sixth Sense has a good example of the key. Bruce Willis' character keeps opening a door with a red knob with a key in his pocket. The key in a story, whether it's an actual key or a key to a mental barrier, gives the user passage.