A prompt that guides the user to take an action, like "Buy now" or "Contact us".
A CTA (Call to Action) is the element — usually a button or phrase — that guides the user toward the next action: "Buy now", "Request a quote", "Message us on WhatsApp".
A good CTA is clear, visible and consistent with the page's context. Its job is to reduce friction between the visitor's intent and the next step — which makes it an important piece at any conversion point.