A subcontractor ("sub") is a specialty firm the general contractor hires to perform a specific trade — concrete, electrical, mechanical, drywall, glazing. Each holds a subcontract (a commitment) with the GC, not with the owner, and is responsible for their scope's labor, materials, and often their own second-tier subs and suppliers.
Managing subs — buying them out, coordinating their sequence, reviewing their submittals and pay apps, and holding them to schedule and quality — is the day-to-day substance of a project manager's job.