I presume you know the difference between subcontracting and outsourcing already.
So basically global subcontracting is employing a person or a business from overseas to work as external providers of service for a business for a time period specified by the contract. By looking globally the business can access an increased pool of applicants as well as take advantage of different working condition/wage regulations. Resultantly, global subcontracting brings cost advantages and can also improve the quality or productivity of the businesses services as opposed to subcontracting domestically.