This is the 2367th most frequent Japanese word.
キー
"Key" (e.g., a physical key, a computer key, or a significant point).
In this sentence, 'キー' refers to the essential factor or important element needed to solve the problem.
この問題の解決のキーは協力です。
The key to solving this problem is cooperation.
Here, 'キー' indicates a physical key used for locking and unlocking the car.
彼は自動車のキーを紛失してしまった。
He has lost the car key.
In this case, 'キー' represents a piano key, part of the instrument used for producing notes.