Cargo refers to goods carried by a large vehicle, like a plane, ship, train, or truck. See a giant truck on the highway piled high with boxes, lumber, or new cars? It's carrying cargo.
Cargo originates from the Latin word carricare which means "to load on a cart, or wagon." Cargo can be loaded on a cart, but it's usually loaded on something much bigger. On a ship, cargo is stacked up in huge, colorful metal containers. These containers can be full of all types of cargo, like food, furniture, or electronics. You'll rarely need the plural, but it's formed by adding an -s or more commonly, an -es (cargoes).