Imagine a car. It's composed from a variety of parts: the wheels, engine, battery, frame, and so on. Some of these parts are also composed of parts: the engine has a block, cylinders, pistons, spark plugs, and so on.
Each part in this ``component assembly" can be a part of only one larger part: its parent component. The hubcaps are part of the wheels, and so they can't also be part of the engine. (They wouldn't fit there in any case, but that's beside the point.) Consider that screws or bolts may be used in many parts of the car: each is part of only one other part of the car, although more than one of the same kind of part may be used in other places. In the UML (Unified Modelling Language) and in PEAK, this kind of parent-child ``assembly" relationship is called composition: a component is being ``composed" by assembling other components.
But in the UML and in real life, this isn't the only way of building things with components. It would be very inefficient if every light and accessory in your car had to have its own, independent electrical system. Ways of sharing components are needed. In the car, wires, pipes, hoses, and shafts serve to connect the services provided by shared components to the places where they are needed. Note that such connections may be between components at any level: wires carry electricity to every electrical part, no matter how big or small. In some cases, wires go to a major subsystem, which then has internal wires to carry electricity inward to its parts, or to carry signals between its parts. In the UML, these kind of ``shared" or ``peer-to-peer" connections are called associations.