A sticky header stays pinned to the top of the browser window as the visitor scrolls down the page. On landing pages, it typically contains a logo, maybe a minimal navigation, and a CTA button. The purpose: no matter how far down the page a visitor scrolls, the primary action is always one click away.
Sticky headers are especially valuable on long-form landing pages. A visitor convinced by a testimonial in the middle of a 5,000-pixel page shouldn't have to scroll back to the top to find the sign-up button. A sticky CTA button solves this elegantly. Data from several A/B tests shows sticky CTAs can improve conversion rates by 5-15% on long pages, simply by reducing the effort between "I'm convinced" and "I'll sign up."
Getting sticky headers right
The biggest problem: sticky headers that eat too much vertical space. A 120px-tall sticky header on a mobile phone (with 700px of viewport) steals 17% of the visible content area on every scroll position. That's a terrible tradeoff. Keep sticky headers thin — 50-60px max on desktop, 44-50px on mobile. Many good implementations use a "smart" sticky that hides when scrolling down and reappears when scrolling up, reclaiming that space.
Also watch the visual weight. A sticky header with a bright background color and a high-contrast CTA button competes with your page content for attention. Consider making the sticky header semi-transparent or using a subtle background that doesn't dominate. The CTA should be visible but shouldn't be screaming at the visitor on every single scroll position.