The phrase "upper rack" refers to the higher level or shelf in a storage area, often used for placing or organizing things above the lower rack. Full definition
Add onion, carrots, and celery to pan and roast on upper rack for 20 minutes. (bonappetit.com)
Bake on upper rack until cheese is browned and sauce is bubbling, 10 to 15 minutes. (styleathome.com)
Roast on upper rack, turning pieces once halfway through with a spatula, until browned, crispy - edged, and cooked through, 15 — 20 minutes. (epicurious.com)