What is the `useEffect` hook in React and when do you use it?

Imagine `useEffect` as a sticky note you leave for yourself on the front door: 'When I get home, do X.' The component renders (you arrive home), and then React reads your sticky note and performs the effect. If you have a cleanup step, it's another note: 'Before you leave again, undo X first.' The dependency array is the condition on the note: 'only do this if certain things changed since last time.'