The phrase "heavy carts" means that the cart is carrying a lot of weight or is difficult to push or pull because it is loaded with heavy items. Full definition
• Provide customers with assistance by pushing heavy carts to their vehicles in the parking lot, and placing their purchases into them. (coverlettersandresume.com)
They were trained to manage cattle and to pull heavy carts for their owners. (blackdogrescueproject.com)
«David manages the «loaner carts» of computers that schools use for state «Smarter Balanced» testing - he keeps track of and maintains the carts and devices, delivering heavy carts on a very tight schedule, and makes the state testing go much smoother for all SFUSD schools. (sfusd.edu)