can not be broken down into smaller or simpler machines/parts

can not be broken down into smaller or simpler machines/parts. A compound machines is a machine made up of simple machines

Compound machines are by definition a compounding or adding of machines. What this means is that if you have a machine, say a bicycle which is made of pulleys, wheels, and levers, you are able to identify that this machine is actually 3 simple machines put together. So if we broke the bicycle down into a pulley, a wheel and a lever, we now have 3 simple machines that can not be broken down any farther. A pulley is just a pulley, a wheel is just a wheel, and a lever is just a lever. There are no other simple machines hiding inside of them. Hence they are simple machines