State with useState

Props come from outside a component. State is data a component owns and can change: a counter's value, whether a menu is open, what's typed in a box.

A plain variable won't work:

let count = 0; // resets on every render, count++; // and changing it doesn't re-render

React needs two things: a value that survives between renders, and a way to tell React it changed so it can redraw. The useState hook gives you both.