A key to a home can be a very important symbol as well as a useful object. Most households have locks that use keys to open them on the external doors. Some homes will also have locked spaces that require a key -- such as a lock on a medicine cabinet, gun safe or gate for a fence protecting people from being harmed in a pool. And people in many homes have car keys too.
Since keys have a way of getting lost or misplaced, it's usually a good idea to have a spare key or spare set of keys put away in a safe place. You may want to ask a neighbor you trust to keep a key to your home, or let you hide it on their property. (So many people hide keys under their own doormats or above the sill that that is not really a good hiding place, but under the neighbor's doormat may be more secure.)
It can be convenient if all the locks can be re-keyed to use the same key.
An alternative to a keyed lock is one that uses a combination -- either the older turn style, levers or with a keypad.
Phrases with the work key tend to denote something that is critical to the situation, including:
- key to my heart
- keynote speaker
- key to the city
- Wikipedia's article on key (lock)
- Wikipedia's article on lock bumping