A system is a set of interrelated components which have emergent behaviour or property.
A car is a system because a car can move you from A to B, but none of its components can individually do that.
Building a system from the 'best' individual components doesn't necessarily result in the best system, because the performance of the system is dependent on the interaction of the components, rather from their individual performances. The performance of the system is an emergent property not found in any of its components.