Products-completed operations is the segment of a CGL policy that responds to claims arising once the work is done, a deck that collapses a year after a sub built it, or a faulty install that causes a fire later. It carries its own aggregate limit.
For GCs this matters most after project closeout. Additional insured status for completed operations (via an endorsement like CG 20 37) is what keeps you protected for work a sub finished long after they have left the site.