A Call to Action (CTA) is a marketing term to define a designed element that solicits and encourages an action from the user and which the end goal is to attempt a sale. You'll hear a lot of this term when you start working as a designer when designing websites, especially for marketing purposes. The goal of every designer is to maximize the click conversion on this button, which leads to a sale at the end. Here is some indication of good practice when designing your CTA.
One piece of advice I'll give is not to be too creative when designing a CTA because it remains a button, and people are used to it. As users have become accustomed to the online experience, they know that CTAs come in the forms of buttons. They see a button; they know what to do. Simple. Make it big, obvious, and stand out from everything around it, and it's in the bag.
Here's an example of a bad CTA:
Image from Capgemini.com website 2017. All rights reserved to Capgemini
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