Without context, I would assume it was referring to something that a person (or idea or corporation or... something) does well (a strength) and identifying it as being particularly important (key).
We'd like to hire him. His key strength is that he already has a strong relationship with the customers we are trying to reach.
However, if you are talking about laptop keyboards or piano keys... well... what WAS being talked about?
What is "key strength"? 
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