A CTA (call to action) is the element of a page that directly invites the user to act: an "Add to basket" button, a "Book a call" link or a sign-up form. A good CTA says exactly what will happen after the click.
A good CTA is specific and honest: it uses a verb to say what will happen ("Download the price list", "Book a call"), and after the click exactly that happens. Vague labels like "Learn more" perform worse because they carry no promise.
Rules that consistently hold up in testing: one primary CTA per screen (competing buttons split attention), clear visual contrast against the rest of the page, and surroundings that answer doubts before the click (price, delivery time, "no obligation"). In e-commerce the CTA path runs through the whole funnel: from "Add to basket" to "Order and pay".
Why it matters for business: the CTA is where interest turns into action, which makes it one of the most rewarding elements for A/B testing. Changes to a button's wording and context regularly produce measurable differences in conversion without rebuilding the page.