React
A JavaScript library by Meta for building user interfaces out of reusable components that update automatically when data changes.
Analogy
Like building with snap-together LEGO pieces instead of carving each wall by hand.
Why it matters
It's the foundation of Next.js and the most in-demand frontend skill, so it underpins most custom apps you'll build.
In practice
A <PriceCard /> component reused across a pricing page, each fed different data.