What does 'rotomolded' actually mean in a cooler?

Rotomolding is a one-piece, seamless plastic construction method that produces thick, uniform walls and rugged corners, which is why rotomolded coolers like the YETI Tundra and Pelican lines are prized for durability and long ice retention. Note that some popular coolers sold as alternatives, including RTIC's Ultra-Light line, are explicitly NOT rotomolded and are marketed as lighter substitutes.