You know what a key is, don’t you? It’s something that opens up a lock. If something is locked, you need a key to open it. But you can also use the word "key" as an adjective or as a noun to describe something that is very important or necessary:
Bill has become a key member of the organization. (In this sentence, "key" is an adjective.)
Dan is a key player on our team. Without him, we might lose.
A key part of the machine is missing, so it won’t work until we find it.
What’s the key to her success? (In this question, "key" is a noun.)
A good education is a key to success in life.
Understanding how the verb "be" changes is a key part of understanding how to form the passive voice.
