useMemo not explained in Tutorial

[MightyPork]

The Context part of the tutorial uses the useMemo() hook, but there's no explanation what this does and how it's used.

Later you are asked to use it yourself when solving the exercise. I still didn't understand what it does, even after reading the solution.

[JoviDeCroock]

That's valuable feedback thank you! useMemo returns a stable reference for a given value, so it can be used to run expensive computations, .... and only re-calculate when the dependencies contained within the second argument (the array) change.