What is React.js?

The model is that your interface is a function of your data. You describe what the screen should look like for a given state, and React works out the minimum set of DOM changes needed to get there when that state changes. You stop writing instructions that manually update elements, which is where most bugs in older frontend code lived.

Because it is a library rather than a framework, everything around the components is your decision: routing, data fetching, form handling, build tooling, and where server rendering happens. That flexibility is genuinely useful in an existing codebase and a liability in a new one, since each of those choices is a chance to pick something unmaintained. Reaching for a framework on top removes most of those decisions.