A Call to Action is a term used to describe a method people use to get someone to do a specific action.
I find the best way to describe what a call to action is, is to use an example.
So, for example if I was selling you something on my site, I may use the Call To Action of “buy now” to try and encourage you to buy my product. It is usually a catchy phrase used to grab the user’s attention to try and get them to perform the task you have the website for. Most companies use it to try and increase their performance and encourage more people to interact with them.