A call to action (CTA) is a specific instruction meant to prompt a visitor to perform a specific task. Usually in the form of a banner, button, or some type of graphic or text, they are used to drive visitors down a conversion funnel. Calls to action might lead to a purchase path or a sign-up or download page but the instruction should be simple, short and very clear.
Well written calls to action will work better than others and an understanding of user behaviour and context can inform the exact wording. Testing different calls to action using multi-variant testing is a good way to optimise them, increase conversion rates and reduce bounce rates.