A web page does not exist without reason. It exists to communicate something and, most often, to direct the user to a desired action.
Adding a product to the shopping cart, subscribing to a newsletter, navigating to another page, making contact with customer service, ... Actions are as varied as the objectives of the businesses using them. But all actions are called "Call-to-Action" (CTA) and are typically represented by a clickable button, a call-to-action button.