What is a “call to action” on a site?

Call to action in web design — and in user experience (UX) in particular — is a term used for elements in a web page that solicit an action from the user. The most popular manifestation of call to action in web interfaces comes in the form of clickable buttons that when clicked, perform an action (e.g. “Buy this now!”) or lead to a web page with additional information (e.g. “Learn more…”) that asks the user to take action.
Call to Action Buttons: Examples and Best Practices by Smashing Magazine

It is literally a visual hint (or shout) that encourages a user to click or take some form of action.

Calls to action are used to:
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