Links are votes of confidence
The idea that made Google is deceptively simple: a link from one website to another is like a vote of confidence. When a respected site links to yours, it's effectively vouching for you — telling both Google and its own readers that your content is worth visiting. The more of these votes you earn from credible sources, the more authoritative Google considers your site, and the higher it's willing to rank you.
Link building is the practice of earning those votes. It's the heart of off-page SEO — the part of your reputation that lives out on the wider web rather than on your own pages. You can have flawless on-page optimization and perfect technical SEO, but in a competitive market, links are often the deciding factor that pushes one site above another. They're the trust you can't build on your own site alone.