The term CTA stands for "Call to Action" and is often used in UX and marketing.
A CTA should stimulate the user to perform a desired action.
In a mobile app, you can find CTAs e.g. in mobile ads, in e-commerce apps, on login screens.
CTAs often have a design that stands (e.g. contrasting colors and bold letters). A good CTA should never go unnoticed and should be clearly recognizable for the user. Besides the design, it should be clear and easy to understand.
CTAs are used to address users directly and give them the decisive impulse to buy a product, sign in or take another intended action.
They help you to move users down the conversion funnel. Without a CTA, users might not know which step to take next or where to finish a task.