React is a framework with a number of moving parts, components, states, props, etc. It can be very easy to begin writing components without properly understanding how each of these should be used and how to use them.
An Engineer who has passed this core skill would be expected to understand what each of these parts are, and when to use one over the other (i.e. When should you put something in a prop instead of the state).
The first two sections of React’s documentation, JSX and Rendering are important reading.