A headless browser is a real browser (Chromium, Firefox, WebKit) run without a visible UI, controlled programmatically via a driver like Playwright, Puppeteer, or Selenium.
Headless browsers are used for automated testing, scraping JavaScript-rendered sites, PDF generation, and SSR smoke tests. They are slow and memory-hungry compared to raw HTTP, but necessary whenever a site loads data via fetch/XHR after initial HTML.