A Call to Action or CTA is a specific area where you are trying to get your visitor to do something. That something may be signing up for a mailing list or purchasing a product. In fact, product pages almost always have a CTA, the “buy now” or “add to cart” button. If you want your visitor to call you, the CTA could be a phone number. It could be a button to “Schedule Your Free Consultation Now” or “Get 20% Off Your First Visit”.
Some areas of a website cry out for a CTA, because they people tend to look at those areas more. The top right of the page is one example.