key

You normally need a key to open your front door or to lock a suitcase.

It can also be a solution to something:
eg Trusting your partner is the key to a good relationship

As an adjective 'key' has other useful meanings:

'Key' can mean 'most important'

eg  The key thing to remember is......
      The key person in the company is the Finance Director

Other words and expressions including 'key':

'Low key'  If something is done in a low-key way it is done relatively quietly and without much fuss. eg The celebration was surpisingly low key.

'Keynote'  A keynote speaker is often the central or most important speaker at a conference or event. The speaker is often well-known and respected by the audience. A keynote speech or address is typically covers the main themes of a conference. 

'keyed up'  Someone who is keyed up is excited and perhaps nervous eg The athlete was so keyed up about running in the Olympic final, she couldn't sleep well.