Wikipedia gives the best definition:
Call to action (CTA) is a marketing term used extensively in advertising and selling.
It refers to any device designed to prompt an immediate response or encourage an immediate sale. A CTA most often refers to the use of words or phrases that can be incorporated into sales scripts, advertising messages or web pages that encourage consumers to take prompt action.
You are generating traffic to your website, to your Social Media pages, … to all your own-channels.
The question is:
What do you expect these visitors to do?
For every visitor, the action to be undertaken must be extremely clear, without any confusion.
This is the CTA – Call To Action.