@turquoise-- I don't think this term has ever been used by the US. It emerged in relation to China, it's their theory and I don't think it applies to any other nation as of I right now. I could be wrong though.
It all started with China's naval expansion and wanting to guarantee naval ports followed by airfields in the surrounding bodies of water. Diplomatic relations and a stronger military was what was needed to assure this.
As you might know, one of the major issues for China is energy. They're trying to secure their energy for the future. They've started many new endeavors to achieve this, like the alliance with other Global South nations like South Africa and Brazil. The string of pearls strategy makes sure that the sea lines that China depends on for delivery of their energy resources is open and clear for them.
In my view, the whole strategy is about energy-- oil.