How we understand complicated systems is not just about finding the parts, it’s about how they affect each other. Cogs fit together so that one turning moves another, but nodes don’t have to be physically connected to have a relationship. You can have a relationship with your friend that affects how you behave. It isn’t just you (a node) but your relationship with your friend (another node!). Would you behave the same way with somebody you don’t like?
But there are more nodes. You might act one way by yourself, and a different way with your friend, or sibling, or parent, or teacher. How would you behave if they were all in the same room?
The links between nodes – the relationships – will tell you how that system behaves.